Rename frac_sno and frac_sno_eff

It's hard to keep track of the difference between frac_sno and frac_sno_eff. @swensosc suggests the following rename to show their primary / intended purposes in the code; I like his suggestion:

- Rename `frac_sno` to `frac_sno_albedo`

- Rename `frac_sno_eff` to `frac_sno_fluxes`

Note that `frac_sno` is used in a few places that aren't related to albedo / radiation. Looking back at the clm4.0 code (from before `frac_sno_eff` was introduced), I see uses of `frac_sno` in:
- DUSTMod: lnd_frc_mbl
- Biogeophysics1: qred and soilbeta

I haven't checked the latest code to confirm that those are still the only uses of frac_sno, but from talking to @swensosc , I think the intent was that most of the new code that depends on snow cover fraction (for the sake of calculating sub-column fluxes) would use `frac_sno_eff`.

Also, note that there are (or at least were in the past) some associates like `frac_sno => frac_sno_eff`, so we need to be careful when doing this rename. Probably a good first step would be to find those associates and fix them so that the name on the left matches the name on the right.