Few corrections to the Observation Sensitivity driver

[arango]

We are still testing the observation sensitivity driver. We corrected few problem in the mathematical formulation. Many thanks to Andy Moore for testing this driver. This is a tricky driver so we expect more adjustments.

I also upgraded the 4DVAR adjustment of surface forcing. What it is new? We are make the algorithm cleaner and considering additional strategies due the several surface forcing options available in ROMS. Many thanks to Gregoire Broquet for testing this algorithm.

I corrected a bug in mod_netcdf.F when reading a 2D floating-point array. I needed to use my_ncid instead of ncid.