Support adding CMake build for RTE+RRTMGP

[tcmetzger]

The RTE+RRTMGP package is buildable with CMake (tests remain as they are). This will be based on the work already started in https://github.com/earth-system-radiation/rte-rrtmgp/tree/cmake.

[tcmetzger]

3 targets:

• make libs: build libraries This one has the highest priority for CMake!
• make tests build tests (some of them more like unit tests, but others
• make check that compares test results to reference data

=> The main goal is platform compatibility and maintaining compiler flags

For Windows, related issue #63 (comment)

[RobertPincus]

A CMake implementation has been merged into a branch in the RTE+RRTMGP repo. It uses CMake to build on Windows and MacOS (Intel/ARM) (relevant to #63) and includes the tests (#64).