Add Spack-based CI workflow + recipe

[AlexanderRichert-NOAA]

This PR adds a Spack-based CI workflow, including an authoritative copy of the recipe (updated for CMake).

Fixes #112

[AlexanderRichert-NOAA]

@edwardhartnett this is in pretty good shape, and I've tested a bunch of the spack variant related to dependencies. One thing that isn't working is if I enable the Fortran API and/or the library. I'm getting wgrib2/ftn_api/fort_wgrib2.c:43: undefined reference to 'wgrib2'. Any idea what that might be about?

[edwardhartnett]

@webisu any clue as to the build problems @AlexanderRichert-NOAA reports?

@AlexanderRichert-NOAA I think we should get this merged without the broken options, so we have something...

[AlexanderRichert-NOAA]

Okay, just updated. Users won't be able to apply build variants that aren't currently working.

[AlexanderRichert-NOAA]

The one remaining issue I can see is that USE_IPOLATES is no longer getting defined for the preprocessor. Is it your intent to only ever use ip or nothing, and have it set as on/off? If so, I think we need to add something like list(APPEND definitions_list -DUSE_IPOLATES under the if(USE_IPOLATES) block in CMakeLists.txt. If that sounds good I can add it to this PR.