Restore spectral scaling to RRTMGP

[brian-eaton]

What is the feature/what would you like to discuss?

From @brianpm:

I've been asked to re-implement the spectral scaling that we used to use
with RRTMG into the RRTMGP code. The issue is that the solar source
function that RRTMGP uses is based on a different model than the
CMIP-endorsed solar spectrum. There is some concern from the ACOM folks
that the adjustments made between those models have shifted energy in
meaningful ways.

@brianpm provided a version of rad_solar_var.F90 modified to work with RRTMGP. I will implement it and work out the kinks.

@PeterHjortLauritzen has indicated that this is a high priority for cesm3.

Is there anyone in particular you want to be part of this conversation?

No response

Will this change (regression test) answers?

Yes

Will you be implementing this enhancement yourself?

Yes