Update GeocodeCov(AP) to subset RSLC based on the geogrid bounding box (#758)

* add crop capability to GeocodeCov (area projection)

* add crop capability to GeocodeCov (area projection) (2)

* remove commented code

* remove unnecessary comments, whitespaces, simplify code

* fix block reading of the input raster

* substitute C-style cast by static_cast

* fix variable names (block range/azimuth min/max)

* revert changes to _getUpsampledBlock()

* precompute RSLC subsetting using geogrid corners

* revert changes to h5_prep.py

* revert changes to gcov.py

* Add/improve comments in GeocodeCov methods

* pass output parameters by pointer rather than by reference

* pass output parameters by pointer rather than by reference (2)

* pass output parameters by pointer rather than by reference (2)

* add comments; fix returning values when loading entire RSLC

* update comment to explain flag_load_entire_rslc check

* fix error introduced by previous commits

* use loadDemFromProj() margin defaults and separate outputEPSG and DEM proj objects

Author: gshiroma