This PR contains a first go at reworking the regrid_restart_file GCPy module to correctly function with GCHP v14.

Key changes are:

• Adds stretched-grid options to the main entry point:
  • --stretched-grid - whether or not we are regridding to a stretched-grid
  • --stretch-factor - the stretch factor of the target grid, which will be added to the global attributes of the output file as STRETCH_FACTOR
  • --target-latitude - the target latitude of the output stretched-grid, which will be added to the global attributes of the output file as TARGET_LAT
  • --target-longitude - the target longitude of the output stretched-grid, which will be added to the global attributes of the output file as TARGET_LON
  • If --stretched-grid is detected, the program requires all three other stretched-grid options to be passed
• Reworks command-line argument and option parsing:
  • Now uses argparse instead of directly reading sys.argv
  • Includes description, example usage, and help text for the program
• Adds documentation throughout the module

I've re-tested the tool with the following regridding configurations:

• Resizing a C90 grid to C48
• Stretching a C48 grid to C120 with SF 6.0
• Re-stretching a C120 grid with SF 6.0 grid to a C48 grid with SF 2.0
  and unless I've missed something (not unlikely - please check!), it looks ok and GCHP 14.0.2 will run fine with the results.

I don't think this is the final solution for regridding in GCPy, though. This still requires gridspec-create => ESMF_RegridWeightGen => python -m gcpy.regrid_restart_file ... as opposed to just using the GCPy module, which would be ideal. I think it is possible to wrap up the gridspec and ESMF_RegridWeightGen stuff into the module so there is just a single task for users to complete, but I don't have time right now to properly look at it.

Let me know what you think.