Corrected 4DVAR observation operator

[arango]

Eliminated a potential problem in extract_obs.F and ad_extract_obs.F when the observation depth is given in meters (negative value) instead of fractional model level (positive value). Observations shallower or deeper than the model associated variable, will be flagged out and ignored. Users should be carefull when preparing the 4DVar observation NetCDF file. If assimilating satellite SST, assign as a depth the model top level (N(ng), positive value) instead of zero or negative depth values. The depth value provided to altimetry SSH data is irrelevant but it is recommended to use a zero value.

Updated obs_read.F and obs_write.F to use new generic NetCDF interface (read and write routines in mod_netcdf.F). Also corrected couple of bugs and cleaned obsolete code and variables.