RRTMGP needs to accept and use physical constants from host model

[grantfirl]

Following #582, RRTMGP should call its own mo_rrtmgp_constants%init_constants() subroutine during the init phase of the CCPP interface so that it uses a consistent set of physical constants as the rest of a physics suite when called through the CCPP. Note that all public constants defined in mo_rrtmgp_constants should be available from the host (all are currently part of the physcons module in FV3), although not all of them have metadata yet, but that is easily added.

There is no need for a special PR to do this -- it can be done as part of other planned updates when it makes sense to do so.