Corrected few things in obs_depth.m

[arango]

Corrected few problems in script obs_write.m. I also added the capability of changing observation with depth = 0 to be located at the surface level. It is assumed that users may assign zero depth to surface satellite data. In ROMS 4D-Var, all the surface satellite data needs to be located at the surface level in the fractional z-grid system.

All the observation fields in the structure are processed with a cell string (field_list) of dynamical fields. This reduces the amount of coding and facilitates expansion in the future.

Many thanks to Chris Edwards for reporting several bugs in this script.