Improve robustness of MILC HISQ MG interface input file parsing

[weinbe2]

The MILC HISQ MG interface currently implements its own functions for parsing MG param input files in the (hideous) spirit of the old command line interface before CLI11 was added. If possible, the routines for parsing input files should leverage the existing CLI11 support. This could also mean support for new MG features could automatically get added to the MILC HISQ MG interface without having to be added by hand.

[weinbe2]

This depends on #1002 being merged.