rte-rrtmgp dtc/ccpp is out-of-date, and must be updated to work with kind_phys=4

[SamuelTrahanNOAA]

Description

The rte-rrtmgp submodule (dtc/ccpp branch) is out-of-date. Although it will compile with kind_phys=4, it won't work properly. There is a proposed solution, but as of this writing, it hasn't been implemented yet. Code manager @RobertPincus pointed this out when merging earth-system-radiation/rte-rrtmgp#180. See another note from him in the comments.

Edit: The main branch doesn't have the changes needed for single precision support either, as noted below in #RobertPincus's comment.

To Reproduce:

1. Compare main to dtc/ccpp
2. Read issues discussing the necessity of several changes to the kernel to support 32-bit floating-point.
3. Notice those changes aren't in dtc/ccpp

[RobertPincus]

@SamuelTrahanNOAA Note that the main branch does not yet run in single precision either. This is the next major task but likely won't happen until August.

[SamuelTrahanNOAA]

@SamuelTrahanNOAA Note that the main branch does not yet run in single precision either. This is the next major task but likely won't happen until August.

I have updated the description to match this information.