Add/modify sea ice diagnostics sivol, simass_n, siitdthick, sivol_CMOR

[MitchBushuk]

Adds and modifies a number of sea ice diagnostics, namely:

-sivol is the equivalent thickness of sea ice (i.e. the grid-cell mean thickness). The prior sivol diagnostic was incorrect. It was equal to sithick, which is the floe thickness (i.e. the thickness of ice averaged over the ice-covered portion of the grid cell). This change removes the old sivol diagnostic, and renames the sivol_CMOR (which had the correct implementation of sivol) diagnostic to sivol. See Notz et al. (2016) for the SIMIP definitions of these diagnostics.
-The prior simass_n diagnostic is a confusing name, since it is actually the sea ice thickness in each category, not the mass in each category. This diagnostic has been modified such that simass_n is now the ice mass in each category. This now has the property that sum_n simass_n = simass.
-A new diagnostic siitdthick has been added, which is the ice thickness in each category, following the SIMIP variable definition.