(*)Conversion arguments for ice shelf scalar diags

  Revised volume_above_floatation and integrate_over_ice_sheet_area to return
values in scaled units, and added conversion factors to the
register_scalar_field calls in MOM_ice_shelf so that all unit unscaling of
diagnostics occurs via  the diag mediator and not in the code itself.  After
this commit, all of the dimensional scaling factors for diagnostics in the
ice_shelf code occur via conversion factors that are immediately adjacent to the
declaration of the units of those diagnostics, facilitating comparison for
consistency.

  The declared units of one diagnostic, "taub_beta", were revised from "MPa s
m-1" to "MPa yr m-1" to be consistent with its conversion factor, which was also
corrected (essentially by a factor of [Z L-1 ~> 1].  Several missing unit
conversion factors for rates of mass change were also added, and about 15
missing or incorrect units in lines documenting real variables were added or
fixed.

  The input scale factor for the (perhaps unused) variable INPUT_VEL_ICE_SHELF
was corrected from `US%m_s_to_L_T*US%m_to_Z` to just `US%m_s_to_L_T`.

  With these changes, all solutions are bitwise identical, apart from regional
cases with a specified inflow when run with dimensional rescaling.  The ice
shelf diagnostics should now be invariant when dimensional rescaling is applied,
and the units of the ice shelf diagnostics are now all consistent with the
required rescaling factors.