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
  - Scaling: https://portal.opengeospatial.org/files/?artifact_id=54504

Test cases will also be included in msautotest.