Add WRF-Chem Apptainer container definition files

[vweeks]

Summary

• Adds Apptainer .def files for building WRF-Chem 4.7.1 / WPS 4.6.0 containers on AlmaLinux 9, supporting both single-layer (distribution) and multi-layer (development) builds
• Includes all WRF-Chem preprocessing tools (mozbc, anthro_emis, fire_emis, megan_bio_emiss, wesely, exo_coldens)
• Adds Dockerfile equivalent of the full single-layer Apptainer build using multi-stage Docker build
• Adds README with build instructions, HPC (Derecho) usage examples, container layout, and known issues (including WRF 4.7.1 KPP compile bug patches)

Closes #76

Test plan

• Build multi-layer containers (deps → preproc → wrf-chem) and verify each layer succeeds
• Verify WRF-Chem executables are present and functional inside container (wrf.exe, real.exe, preprocessing tools)
• Test running on Derecho with bind mounts and MPI
• Build full single-layer Apptainer container: apptainer build --fakeroot wrf-chem_full_almalinux9.sif wrf-chem_full_almalinux9.def
• Build Docker image: docker build -t wrf-chem:4.7.1 .

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