Make RRTMG++ use Scorpio for reading

YAKL uses C++-Netcdf to read in netcdf files, while the rest of SCREAM uses a Scorpio interface to C-Netcdf. It would be nice to have a single interface for all processes within everything in SCREAM. This is perhaps impossible, however, since the C++-Netcdf calls are part of an external submodule dependency. We'd like to apply pressure to YAKL devs to internally include the C++ to netcdf C interface it needs. Iulian says that CPL already developed a nice solution to this.