Making validation and verification more portable

[RobertPincus]

Verification and validation have grown organically with the code base. At this time this include

• (verification) unit tests for RTE optical properties and solvers in tests
• (verification) invariance tests for gas optics + solvers in tests
• (evaluation) exploration of alternatives (e.g. reduced numbers of g-points, number of quadrature angles) with tests/check_variants
• (validation[^1]) ability to reproduce clear-sky fluxes and compare to benchmarks over a wide range of conditions in examples/rfmip-clear-sky
• (validation[^1]) ability to reproduce all-sky fluxes examples/allsky

[^1]: Requires running make check with an appropriately-configured Python environment.

To prepare for alternative implementations of gas optics we might want to isolate the specific dependencies of the tests and examples on RRTMGP. This has been done for the gas optics in tests by encapsulating the gas optics type and its initialization in mo_gas_optics_defs.F90.

A more general solution looks something like

• [ ] Encapsulating gas, cloud, and aerosol optics -- both types and their load routines -- in a single scheme-specific module. This module probably lives with the scheme (i.e. in rrtmgp/) rather than in the tests directory.
• [ ] Converting the examples and any remaining tests to use the encapsulated optics
• [ ] Possibly a reorganization? tests/check_variants is used to assess accuracy with a) an alternative spectral resolution, if provided, and b) a variety of LW solver options. Does this belong somewhere else?