initial work on an extension to facilitate faceted search

triggered by #52

proposals/aggregations/README.md

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+# OGC API - Records - Term Aggregations
+
+This folder contains the content for the standard extension OGC API - Records - Term Aggregations.
+
+# Overview
+
+This extensions enables the capability to include Term Aggregations in the items (records) response. These aggregations can be used by clients to enable [faceted search](https://en.wikipedia.org/wiki/Faceted_search).
+
+Various backends support faceted search. Examples are [Elastic](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-terms-aggregation.html), [SOLR](https://solr.apache.org/guide/8_8/json-facet-api.html) and limited support in [PostGres](https://akorotkov.github.io/blog/2016/06/17/faceted-search/), [Oracle](https://blogs.oracle.com/apex/apex-192-faceted-search).
+
+Facet statistics are also interesting to give an overview of the spatio temporal distribution of items in a (part of a) collection.
+
+## Extending a json search result with Term Aggregations
+
+The server includes the Term Aggregations for any search result within a collection. For example, for a collection `cat` the response to
+
+```
+GET /collections/cat/items
+```
+
+may include the following property `aggregations`. The content represents an array of aggregated terms.
+A aggregation is identified by a collection property name and contains the top number of buckets with
+their count. A bucket is a number of results in the resultset matching the key. A parameter `next` 
+indicates if there are potentially more buckets to be retrieved. 
+
+```
+{
+  "type": "FeatureCollection",
+  "aggregations": [
+    {
+      "property": "keyword", 
+      "buckets": [
+        {
+          "key": "forestry",
+          "count": "202"
+        },
+        {
+          "key": "marine",
+          "count": "150"
+        }
+      ],
+      "next": "0"    
+    }
+  ],
+  features": [],
+  "numberMatched": 375,
+  "numberReturned": 0,
+  "links": []
+}
+```
+
+## Numerical, spatial and temporal buckets
+
+For dynamic values a range of buckets can be returned defined by a min, max or bbox value. 
+
+```
+{
+  "aggregations": [
+    "key": "scale", 
+    "buckets": [
+      {
+        "key": "0.1 - 0.001",
+        "count": "175"
+      },
+      {
+        "key": "0.001 - 0.00001",
+        "count": "120"
+      }
+    ],
+    "next": "2"
+    },
+    {
+      "key": "date",
+      "buckets": [
+        {
+          "key": "1990/01/01 - 1995/01/01",
+          "count": "12"
+        },
+        {
+          "key": "1995/01/01 - 2000/01/01",
+          "count": "340"
+        },
+        {
+          "key": "2000/01/01 - 2005/01/01",
+          "count": "1200"
+        }
+      ],
+      "next": "5"
+    }  
+  ]
+}
+```
+
+In the case of geometries this is a boundingbox in WGS84 defined by two points. 
+
+```
+{
+  "aggregations": [
+    {
+      "key": "bbox",
+      "buckets": [
+        {
+          "key": "0,0 5,5",
+          "count": 175
+        },
+        {
+          "key": "0,5 5,10",
+          "count": 120
+        },
+        {
+          "key": "10,5 15,10",
+          "count": 77
+        }
+      ],
+      "next": 7
+    }
+  ]
+}
+```
+
+## Interacting with aggregations
+
+In some situations clients are interested only in the aggregations. In other situations the aggregations are not required. These aspects can be controlled via additional query parameters.
+
+| Parameter | Explanation |
+| -- | -- | 
+| aggregationsOnly | default `false`, Returns the aggregations without search results. Similar to `&limit=0`. |
+| includeAggregations | default `true` (if available), can be set to `false` |
+
+# Folder structure
+
+This folder is organized as follows:
+
+* openapi - normative OpenAPI components specified by the standard
+

proposals/aggregations/openapi/parameters/aggregationsOnly.yaml

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+name: aggregationsOnly
+description: parameter can be used to request aggregations only, similar to &limit=0
+in: query
+required: false
+schema:
+  type: string
+  format: uri
+style: form
+explode: false
+default: false

proposals/aggregations/openapi/parameters/includeAggregations.yaml

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+name: includeAggregations
+description: parameter can be set to omit aggregations in search result
+in: query
+required: false
+schema:
+  type: string
+  format: uri
+style: form
+explode: false
+default: true

proposals/aggregations/openapi/schemas/aggregation.yaml

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+Aggregation:
+  type: object
+  description: An aggregation is linked to a dataset property and contains the top set of occurences (buckets) of values of the property
+  required:
+    - key
+  properties:
+    key: 
+      type: string
+      description: Name of the property
+    next:
+      type: number
+      description: Indication how many additional buckets exist for this property
+    buckets:
+      type: array
+      description: buckets for this property
+      items:
+        $ref: "#/definitions/Bucket"
+

proposals/aggregations/openapi/schemas/aggregations.yaml

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+type: array
+description: List of aggregations
+items:
+  $ref: "./aggregation.yaml"

proposals/aggregations/openapi/schemas/bucket.yaml

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+type: object
+description: A bucket is the number of items sharing a certain value for a property
+required:
+   - key
+   - count
+properties:
+   key:
+     type: string
+     description: Bucket value
+   count:
+     type: number
+     description: Number of hits for this value