fix CRS handling in the DescribeCoverage response (refs #21045) [wcs]

[alexbruy]

Description

According to the OGC WCS 1.0.0 specification in the DescribeCoverage response the supportedCRSs element has either a requestResponseCRSs sub-element, or both a requestCRSs sub-element and a responseCRSs sub-element. QGIS WCS provider handles only case when requestResponseCRSs sub-element is present. As result a wrong GetCoverage request constructed later as described in https://issues.qgis.org/issues/21045.

Another related issue is that some WCS servers, for example http://thredds.ucar.edu/thredds/wcs/grib/NCEP/WW3/Regional_US_West_Coast/Best, contain invalid(?) CRS definition EPSG:0 [Latitude_Longitude] in the response elements, e.g

<gml:RectifiedGrid srsName="EPSG:0 [Latitude_Longitude]" dimension="2">

which is used later to construct WCS request.

This PR tries to fix both issues:

1. parsing of the DescribeCoverage now handles either a requestResponseCRSs sub-element, or both a requestCRSs sub-element and a responseCRSs sub-element.
2. CRS definitions from the DescribeCoverage response now checked for validity and invalid definitions are excluded from the coverage summary.

Checklist

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