Unify default parameters in MILC HISQ MG interface with defaults in `utils/set_params.cpp`

[weinbe2]

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The MILC HISQ MG interface currently hard codes a handful of default values for the various MG-related structures. This is largely redundant of the setStaggeredMGInvertParam and setStaggeredMultigridParam functions in utils/set_params.cpp. The MILC HISQ MG interface should instead leverage these functions in some capacity to avoid redundant efforts and improve robustness.

[weinbe2]

At time of filing this depends on #1002 being merged.