Make h2osno purely a diagnostic variable

[billsacks]

Currently, h2osno is used in two ways:

1. When there are explicit snow layers, h2osno is the sum of snow (liquid + ice) in all layers

2. When there is a little bit of snow, but not enough to create a snow layer, h2osno gives the amount of this very thin snow pack (assumed to be all ice)

This is confusing and complicates the addition of water tracers to the code. I am separating h2osno into two different variables:

1. h2osno_unresolved will be a fundamental state variable that gives the amount of snow in the case where we don't have explicit snow layers

2. h2osno_total will be purely a diagnostic variable that is the sum of snow in all explicit layers, plus h2osno_unresolved