feat: add initial MICM gas phase chemistry solver process

[mbruckner-work]

Note this builds on #154.

Adds MICM gas phase chemistry solver to CATChem. Initial scheme is without photolysis (or "dark earth").
Mechanism configuration included in tests is a simple chemical loss of SO2.

[zmoon]

Well it worked on my laptop but in CI it's saying

Error creating MICM: 
Provided chemical system contains no species.

[mbruckner-work]

Ran into the same issue from the test with a new build of the code on Hercules.
The test also needs to get passed the reactions.yaml and phases.yaml that are listed in the micm_config.yaml.

I forgot to push an earlier commit that adds the process to the api. How do I add the guard around it for the build without MUSICA?

[zmoon]

Ran into the same issue from the test with a new build of the code on Hercules. The test also needs to get passed the reactions.yaml and phases.yaml that are listed in the micm_config.yaml.

Ah gotcha, I was actually copying those without knowing when I tested it locally, then I changed to a different method for the commit. I can fix.

I forgot to push an earlier commit that adds the process to the api. How do I add the guard around it for the build without MUSICA?

We need to use preprocessor conditions (#ifdef MUSICA or something) and use the CATCHEM_BUILD_MUSICA setting to conditionally turn it on via cmake target_compile_definitions. But we might want to try to set it up so it's still in the API, but fails with a nice message if MUSICA is going to be used but not available.

Edit: I put in something simple to get it to work for now.