Vertical integral of humidity field

[daw538]

New output diagnostic for spectral dynamics that calculates the total column integrated humidity directly in the model code. Output is in units of kg/m^2, which can easily be converted to a depth by dividing through by the density of the liquid phase. I have sanity checked values with a Earth-like run.

This was originally developed to check conservation of liquid in model but decided it would be a useful diagnostic to make available.

Trip tests pass successfully:

Results for all of the test cases ran comparing 5842293 and d5a4a19 are as follows... axisymmetric : pass bucket_model : pass frierson : pass giant_planet : pass held_suarez : pass MiMA : pass realistic_continents_fixed_sst : pass realistic_continents_variable_qflux : pass socrates_aquaplanet : pass socrates_aquaplanet_cloud : pass top_down_test : pass variable_co2_grey : pass variable_co2_rrtm : pass ape_aquaplanet : pass