gemini3d/gemini3d: introduce libgemini with C or Fortran frontend

As the first step of easing and generalizing access to Gemini3D from other libraries and programs, make a C CLI to complement the Fortran CLI. It's possible that the Fortran CLI will go away to be replaced by C CLI. From user's perspective little would change. More FindHDF5 fixes based on user experiences More MSIS 2.0 fixes--require /msis_version in HDF5 input instead of CLI argument for MSIS version. The CLI seemed to give intermittent all-zero output from MSIS 2.0