Rename `*_ICE` diagnostic fields to `*_GLC`

[billsacks]

Some confusion arose recently connected to the _ICE suffix on CTSM history variables. For most history fields, SOMETHING_ICE means that this is the SOMETHING field but only over glacier landunits. However, there are a few – such as QRUNOFF_ICE_TO_COUPLER – where ICE means that this is an ice as opposed to liquid term. So in this case, this is ice runoff averaged over all landunits.

I proposed, and LIWG members supported, that we change the suffix that denotes that a field is only averaged over glacier landunits, using _GLC rather than _ICE for this purpose.

This will be an easy change that will only affect a handful of history fields (hence I'm giving it the "simple bfb" label). However, it will impact the ability to compare master with CMIP6/ISMIP6 runs, so I'd suggest holding off on this until we're past the point that people will frequently want to do such comparisons with master – e.g., holding off until after water isotopes are in place (hence I'm giving it the "blocked: answer changing" label). (And yes, I recognize that it seems weird to have both of these labels, but both seem appropriate in this case.)

[ekluzek]

@billsacks is this something that still should be done? And is it still blocked by isotopes, since isotopes aren't immenent?

[billsacks]

Nobody has been asking for this, but my sense is that it still seems like something that should be done, and it should be straightforward. No, it's no longer blocked, so I removed that label.