Better documentation and validation/normalization of CRS code usage

[soxofaan]

A Terrascope user got a cryptic error message

...
py4j.protocol.Py4JJavaError: An error occurred while calling z:org.openeo.geotrellis.ProjectedPolygons.fromExtent.
: org.locationtech.proj4j.UnknownAuthorityCodeException: 32616
	at org.locationtech.proj4j.CRSFactory.createFromName(CRSFactory.java:85)
...

after some digging it turned out that the CRS was wrongly encoded in process graph, something like:

      "process_id": "load_collection",
        ...
        "spatial_extent": {
          "west": 300000,
          ...
          "north": 2100000,
          "crs": "32616"
        },

In Terrascope backend we support "crs": 32616 and "crs": "EPSG:32616", but not "crs": "32616".
Apparently we document these subtleties not very good in the openeo python client docs.

It would also be better if we already catch this error client side (so bit of validation/normalization of crs in various places)

[jdries]

extra idea: support pyproj.CRS as input

[JohanKJSchreurs]

This PR below will also help to alleviate this problem:

• PR: Issue259 allow epsg code as string #453
• GH Issue: Allow epsg-code to be provided as string? #259

Nonetheless, we do need to document more clearly how to use the CRS / EPSG codes.

[soxofaan]

can be closed now