Use 10-m wind for snow parameterizations

Wind-dependent snow density should use 10-m wind rather than the atmospheric forcing.

According to @dlawrenncar , @swensosc also pointed out that u10 should probably be used for snow unloading as well.

I started a branch for this work a while ago here:

https://svn-ccsm-models.cgd.ucar.edu/clm2/branches/snow_density_uses_10m_wind/

However: I deferred this due to a problem in driver sequencing: u10_clm is computed mid-way through the driver loop; some uses in the snow code are before this and some are after. So at the very least we would need to move this field to the restart file. But this could cause confusion, because some uses would use the previous time step's value, and some would use the new time step's value.