Change default for clm6_0 physics for use_c13 and use_c14 to be default on

[ekluzek]

Especially for the coupled simulations we need use_c13 and use_c14 to be on. But, we also want to make sure they are on for our own spinup simulations. It might as well be the default for clm6_0, and if users want to save a little bit of compute time they can turn it off by themselves, or use a previous version where it was default off.

[ekluzek]

@wwieder just brought this up again.

If we wanted to make this depend on the compset, we'd probably want to make this an XML variable setting something like:

CLM_C_ISOTOPES=TRUE/FALSE

Which then we'd have set to TRUE when coupled to MOM.

[wwieder]

Looks like this was done in #3268, closing this issue