Physics changes made for/during HFIP 2020

[hannahcbarnes]

• All changes in GF (1. set c0 = 0.02 for mid-clouds. 2. changes water-ice transition temperature to what was used in FIM [transition now occurs at 258 instead of 248K])
• Also does some clean-up for how c0 is used in the code.

[hannahcbarnes]

@DomHeinzeller Here is the pull request Shan and Georg were talking about. Since it only impacts the ccpp-physics repository I only made the request here. Let me know if this change also requires pull requests from fv3atm or the ufs-weather-model repositories.

[climbfuji]

@DomHeinzeller Here is the pull request Shan and Georg were talking about. Since it only impacts the ccpp-physics repository I only made the request here. Let me know if this change also requires pull requests from fv3atm or the ufs-weather-model repositories.

Thanks! I can take care of the fv3atm and ufs-weather-model PRs. Will do that after my next two meetings.

[DomHeinzeller]

Associated PRs:
#50
NOAA-GSL/fv3atm#45
NOAA-GSL/ufs-weather-model#37

For regression testing information, see NOAA-GSL/ufs-weather-model#37.