Adding support for Generic EPSG codes via GDAL

[piyushrpt]

Not sure if you are accepting PRs but here is a relatively minor PR that should not impact any of the processing code currently in use. No rush to incorporate it - can be merged in at a later date.

1. Adds a new GenericEPSG class to isce::core to support transformations using EPSG codes.
2. Mostly useful for working with various national-level equal area projection systems
3. CUDA code should still error out when these EPSG codes are passed in - provides CPU-only support.
4. Includes unit tests for some countries - accuracy of transformations is completely dependent on implementation in GDAL / PROJ.