`h5_to_gdx.py`: Add backwards-compatibility for older GAMS versions

[patrickbrown4]

Summary

#61 inadvertently dropped compatibility with older versions of GAMS due to the new use of $loadDCR. We'd like to keep ReEDS working with older GAMS licenses, so it would be good to add a workaround for users with older GAMS versions.

We want to support GAMS versions at least back to 35.1.0.

Proposed approach

• runreeds.py
  • Check the GAMS version and include it as a switch in this block (like the hpc switch)
• h5_to_gdx.py
  • For older versions, instead of writing the parameters to inputs_0.gdx, either:
    • switch to csv
      • write them as .csv files to inputs_case/gams
      • Replace write_declaration() and write_gdxread() with new .gms scripts in the autocode folder, resurrecting the approach from https://github.nrel.gov/ReEDS/ReEDS-2.0/pull/1103
    • OR figure out a simpler approach (just need to define before $loadDCR, and/or switch to $load?
• b_inputs.gms
  • Wrap this block in the new GAMS version switch. Use the current code for newer versions. For older versions, use the alternative approach from the prior bullet.