Renamed flag OBS_SENSITIVITY

[arango]

For consistency and clarity with other 4D-Var observation senstivity algorithms, the C-preprocessing option OBS_SENSITIVITY was renamed to IS4DVAR_SENSITIVITY. The driver for this option was also renamed from obs_sen_ocean.h to obs_sen_is4dvar.h. Very small changes were made to several files to account for the new CPP flag.

I also made few changes to routine netcdf_var_check in module mod_netcdf.F to check more carefully the 4D-Var parameters. Only the ones needed are checked. Many thanks to Andy Moore for reporting this potential problem.

Corrected a bug in the balance operator when integrating the hydrostatic equation. We need a 1/rho0 factor instead of g/rho0 here. Many thanks to Andy Moore for bringing this to my attention.