Abstraction layer for SensorThings API query and filter functions

[timEngbrocks]

SensorThings API: query and filter functions

The SensorThings API offers many built-in query and filter functions for queries to the SensorThings Service.
Some examples:

• st_within: Find things in an area
• eq: Find things where some property equals some value
• distance: Calculate the distance between two points

The full documentation can be found here: OGC SensorThings API Part 1: Sensing in section 9.3.3.5

Current state

Currently it is possible to add filters to queries by calling filter() on a query: method definition
This method takes a string which is placed into the query which means that part of the query has to be defined manually.

Suggestion

It would be great if filters could be controlled with built-in methods.
A query could for example have a list of filters where each filter has a operation and a list of arguments.
This would provide a layer of abstraction so that users of the FROST-Client don't have to know the SensorThings API Syntax.

[hylkevds]

Yes, this would be very nice to have, but also a lot of work to implement.

[hylkevds]

This is also related to #37, since Filters can be used in Expands...