Add low level functions for clouds via pybind

[tcmetzger]

Additional low-level functions related to clouds and aerosols as defined in the C++ header files in the RTE+RRTMGP repository ( https://github.com/earth-system-radiation/rte-rrtmgp/blob/develop/rrtmgp-kernels/api/rrtmgp_kernels.h and https://github.com/earth-system-radiation/rte-rrtmgp/blob/develop/rte-kernels/api/rte_kernels.h) are available in Python, using pybind and the workflow established in Phase 1.

[RobertPincus]

@tcmetzger I made a start on this with earth-system-radiation/rte-rrtmgp#293 where I separated the cloud kernels. (The aerosol kernels being maybe too much of a mess.) When I made the change, though, one of my CI tests started to fail (GPUs using double precision, with the same code in single precision working fine.) I haven't resolved this but can come back to it.

[tcmetzger]

In this phase, we won't do aerosols or only as a s stretch goal, if at all!

[RobertPincus]

@tcmetzger @makepath-alex @sehnem C++ header files for cloud optics kernels are available on develop as of 06ea828
.