Add find-package modules for GPTL, PnetCDF, udunits (unify JCSDA and EMC cmakemodules)

[climbfuji]

My goal is to unify the JCSDA and EMC CMakeModules, and for this I need to add a few find-package modules that exist in https://github.com/jcsda/jedi-cmake to this repository:

• GPTL
• PnetCDF (not sure if this is the right capitalization, but that's what it is called in jedi-cmake)
• udunits (should this all be uppercase?)

Almost all of the existing files are identical between the two repos. There are tiny differences between the FindNetCDF.cmake files, but they are just additional comments and don't matter.

There are, however, large differences between the FindPIO.cmake files. My strategy is to test first if the version in this repository (CMakeModules) work just fine, and if so not care about the differences. If it doesn't, come back and revisit the FindPIO.cmake differences in a follow-up PR.

Question

Do the copyright statements at the top of the new files matter? I assume not, since the existing FindNetCDF.cmake file has an ECMWF copyright statement at the top.

Testing

This PR needs to be tested with jedi-cmake (which will be using this repo as a submodule). Requires installing a test version of jedi-cmake using spack-stack and then running our JEDI tests (with MPAS, FV3, ...).

Issues

First step (maybe the only step required) towards #65

[aerorahul]

Looks good.

[climbfuji]

Update: I updated jedi-cmake (make CMakeModules a submodule and delete the existing findXYZ.make files) and tested it in several ways on my macOS:

1. Build jedi-bundle with jedi-cmake being installed via spack-stack and loaded as module
2. Build jedi-bundle with jedi-cmake NOT being installed via spack-stack and loaded as module, i.e. it gets built as part of the bundle
3. Build mpas-bundle with jedi-cmake being installed via spack-stack and loaded as module (mainly for testing if the new findPIO.cmake works).

All of these worked fine, including finding PIO. I think we can merge this.

[climbfuji]

@kgerheiser Do you want to take a look?

[kgerheiser]

Looks good