Decide on how depth, surface_area and volume should be handled in NSM

[sjordan29]

See the discussion under #78 (comment)

Need to decide how depth should be defined in NSM.

[imscw95]

It doesn't look like surface_area and volume are direct inputs into NSM, and depth is currently just a static variable. Since the value for these 3 variables will change based on the grid cell - similar to the state variable concentrations - it makes sense to treat them like the state variables. Since NSM only needs depth as a direct function input, we can grab that directly if the hydro model keeps track of that value. However, TSM uses surface_area as a direct function input, so we will have to grab that variable for TSM anyway. But I'd think the linkage between TSM and NSM to the hydro model would be separate, since to me that linkage is all about passing the state variable dictionary back and forth, and the two modules will have different state variable dictionaries.

To me it comes down to what is easiest to track in the hydro model.