Major API simplification

This uses xarray.Dataset() to drastically simplify the API. Everything is packed in a 3-D, 4-D (energy vs. alt vs. params) or 5-D (time vs. energy vs. alt vs. params) Dataset, with several data variables in the Dataset.

The glowaurora prereqs have had their own prereqs trimmed so that the install is simple.
The only Python extension module compiling is for Glow itself.

This lays groundwork for new GLOW modern Fortran code integration.