GAMMA handling improvements

[johntruckenbrodt]

This enhances the usability of the gamma module:

• GAMMA's LAT module is no longer needed: new pyroSAR-internal implementations can be used if the module is missing (concerns commands product, ratio and linear_to_dB)
• improved backwards compatibility: new cases have been added in which pyroSAR uses the newest command if usable and falls back to an older alternative if not (multi_look_ScanSAR->multi_S1_TOPS, gc_map2 does not provide all needed arguments); an additional check addresses the case that gc_map might not have an argument OFF_par
• gamma.pixel_area_wrap: new argument exist_ok to run the function in update mode; eventually this argument will be introduced in gamma.geocode as well
• various bug fixes
  • gamma.convert2gamma: raise an error if S1_bnr=True but the GAMMA command does not support border noise removal
  • gamma.geocode: removed unneeded underscore in HDR file naming
  • examine.ExamineGamma: search for gdal-config command instead of a hard-coded path
  • gamma.pixel_area_wrap: fixed some issues with occasionally missing intermediate files, e.g. for computing ratios
• documentation improvements