(*)Correct SIS summed output, including salt units

  Corrected several dimensional consistency or dimensional documentation issues
in SIS_sum_output.F90, and generally simplified the summed output code:

 - Corrected the documented units of a several salt content variables

 - Corrected the magnitude of the diagnostic salt content stocks written to
   stdout, which had been off by 3 orders of magnitude (i.e., were in tons of
   salt) relative to what might have been expected (kg of salt).

 - Consolidated the dimensional rescaling factors into just 3 variables for the
   rescaling of areas, area integrated masses, and area integrated heat

 - Introduced separate variables for the cell-area integrated heat, water and
   salt inputs in mks units to differentiate them from the dimensionally
   rescaled variables used to accumulate column inputs per unit area

 - Removed several unused variables, especially in accumulate_input_1

  All solutions are bitwise identical, but there are changes to the output to
stdout if WRITE_STOCKS is true.