CMake refinements

[RobertPincus]

Issues noted by @skosukhin outstanding from #316

• Come up with a common prefix for custom CMake variables. For example, BUILD_TESTING is a standard variable and should keep the name but KERNEL_MODE, FAILURE_THRESHOLD, RTE_ENABLE_SP, etc. are project-specific and should have the same and a project-specific prefix. Usually, this would be the name of the project but RTE_RRTMGP_ seems to be rather long. Maybe we could use something like RR_.
• The tests are currently missing dependencies on each other and, therefore, we cannot run them in parallel.
• Introduce the missing test FIXTURES and consider fetching the required data at the build time as one of the tests.
• Make it possible to install the libraries (and maybe the data files).
• Come up with the public aliases for the targets (e.g. rte-rrtmgp:rte, rte-rrtmgp:rte-dp, etc.).
• Make sure we can build rte-rrtmgp as a subproject (i.e. add_subdirectory(rte-rrtmgp) should work and expose the targets).
• Make sure that a superproject can find rte-rrtmgp (i.e. find_project(rte-rrtmgp)) in the build directory, as well as in the installation prefix.
• Consider changing the directory layout. For example, it feels natural to move rte-kernels and rte-frontend to a new first-level subdirectory rte. Directories gas-optics, rrtmgp-kernels and rrtmgp-frontend could be moved to a new first-level subdirectory rrtmgp.
• We also have a minor inconsistency: the kernel mode is named extern but the respective files are stored in the api subdirectory.
• Revisit .gitignore.
• Extend formatting jobs to cover Python (isort, black, flake8), C/C++/CUDA (cmake-format) and Fortran (fprettify) source files.
• Remove setup.sh (unconventional and misleading) and provide the building instructions in README.md (or elsewhere)

[RobertPincus]

A note on the directory layout: gas_optics belongs to rte; rrtmgp is one implementation.

Also rte and rrtmgp may become separate repos in the not-too-distant future.

[RobertPincus]

Two issues resolved in #319

[RobertPincus]

Four issues resolved by #323