Snow density unrealistically large for very thin snowpacks

### Brief summary of bug
Snow densities derived from ctsm history output of H2OSNO and SNOWDP can be unrealistically large (>1000kg/m3)

### General bug information
Calculating snow density as rho=h2osno/snowdp from time step frequency history files shows very large values at times when the snowpack is very thin. This typically occurs when the snowpack is unresolved, i.e. snl is 0 but h2osno_no_layers > 0. The issue appears to happen because frac_sno (snow cover fraction) is held fixed throughout the timestep, while h2osno and snow_depth are updated due to phase change and frost deposition. I corrected the behavior by making changes to how snow_depth is calculated during phase change, and using an updated value of frac_sno in the calculation of snowdp.
**CTSM version you are using:**
**Does this bug cause significantly incorrect results in the model's science?** ?

**Configurations affected:**