WCS20 extensions

When WCS 2.0 was implemented in MapServer, much of the functionality of previous WCS versions was not standardized, due to the Core/Extension model of the WCS 2.0 specification. Now some extensions are finally ratified or about to be ratified. 

This pull request implements the following WCS 2.0 Extensions:
  - Interpolation: https://portal.opengeospatial.org/files/?artifact_id=54502
  - Range Subsetting: https://portal.opengeospatial.org/files/?artifact_id=54503
  - GeoTIFF (link not yet available)
  - CRS: https://portal.opengeospatial.org/files/?artifact_id=54209

The Scaling extension would not be backward compatible to our previous implementation. So it is yet to be decided if and how it will be integrated.

Test cases will also be included in msautotest.