Rename l2g_scale_type and make its values integer constants

Connected with #1297 there are a couple of changes I'd like to make to l2g_scale_type before we start applying it more widely:

- [ ] Rename it: It's not really a scaling. @ekluzek suggests the name `landunit_mask`, and I like that
- [ ] Following our general design principle, I'd like to change the string values ('veg', 'ice', etc.) to integer constants
- [ ] Then I think we can simplify some code (or at least change / remove some comments) in subgridAveMod: assume this is a mask rather than a scaling. While we're at it, it might be good to optimize this code so that we precompute and store the necessary mask arrays in initialization, rather than recomputing them every time they're needed. However, it may be worth deferring this piece, since we might end up getting rid of a bunch of this code in subgridAveMod if we take the approach outlined here: https://github.com/ESCOMP/CTSM/issues/1344#issuecomment-824259091