Add a time-dependent function for CO2 (Update PR #1173)

[jihyeonjang]

This PR builds on #1173, which proposed a smoothly increasing, time-dependent CO2 concentration function. The function form is a close fit for past CO2 growth and extrapolates it for the future.

This update introduces a new radiation utility module for subroutines or functions used by both LW and SW radiation schemes.

Summary of changes:

• rebase PR Make CO2 a function of year instead of a single constant #1173 changes onto the current develop branch
• add a new utility module (mpas_atmphys_radiation_utils.F)
• move radconst and co2_estimate into mpas_atmphys_radiation_utils
• rename co2 to co2_val to avoid conflict with the non-1D co2 array used by the CAM radiation scheme
• compute co2_val in radconst
• call radconst before rrtmg_lwrad and rrtmg_swrad
  (Moving radconst from mpas_atmphys_driver_radiation_sw.F90 to mpas_atmphys_radiation_utils.F90 removes the build dependency of LW on the SW module.)

A future update will implement reading GHG values from prescribed tables, following the WRF approach. The table year-dependent GHG is based on observed values for past years and official climate scenarios for future years.

[weiwangncar]

@jihyeonjang Creating a separate module for radiation utility is good. I would prefer to have CO2 computed from a different routine than radconst. This will keep radconst the same as in WRF, and later when a table version of CO2 is added, no change would be required to modify radconst. But I'm ok if you and Michael are ok with the current solution.

[dudhia]

I had similar thoughts to Wei as CO2 and radconst are not physically related.

[jihyeonjang]

Thank you, Wei and Jimy, for your comments. I thought radconst is a subroutine that produces constants for the radiation scheme, which is why I added the CO2 function in radconst. But I agree with Wei's suggestion. As she mentioned, to keep radconst the same as in WRF, I will modify the code so that the CO2 function is handled seperately from radconst.

[weiwangncar]

@jihyeonjang Thanks for improving my old PR.

[jihyeonjang]

@weiwangncar Thank you so much for the quick review!