HyP3-lib v1.2.1

Added:

• DemError, ExecuteError, and GeometryError (subclasses of the generic Exception) for
  more targeted error handling

Fixed:

• Updated the ps2dem.py to handle the GAMMA 2019 create_dem_par interface
• Removed GIMP, REMA, and EU DEMs from the default config (Note: they are still available
  by editing the default config) due to many failures associated with these DEMs
• No library functions will raise a SystemExit and will some subclass of Exception, as
  would be expected of a library, so that errors can be handled by the calling app
• execute raises a hyp3lib.ExecuteError instead of a generic Exception for more targeted
  error handling
• get_asf.py will not fail silently anymore.