Ensure that invalid cells and levels are populated with FillValue

[cbegeman]

Currently, SpecVol is initialized with all zeros and those zeros persist in invalid cells and levels where SpecVol is not defined. We should make sure SpecVol is populated with the FillValue in those places.

[xylar]

If I'm not mistaken, no fields are getting initialized with fill values. I'm seeing (in the overflow) that temperature values from the Polaris initial condition persist for the full duration of the simulation; zero values for layer thickness are also seen below the topography, rather than fill values.

Just pointing out that we may have a broader issue with needing fill values handled in Omega in a proper way.