obs_converter notes: obs_err mods

[hkershaw-brown]

obs_error directory

1. Duplicate file meteor_mod.f90:
   obs_converters/NCEP/netcdf/meteor_mod.f90
   obs_converters/MADIS/meteor_mod.f90

2. obs_converters/obs_error directory:

The readme

IMPORTANT:
Each file should have the same module name; i.e. the source
file names will differ but the module name inside the file
must be the same across all modules in this directory.

All the subroutines must also have the same names and
calling sequence. They must return appropriate values 
for each observation type that is required.  
If errors for a new observation type is added, it should
be added to all the files in this directory.

ncep_obs_err_mod.f90 has more public routines than ecmwf_obs_err_mod.f90

public :: metar_pres_error,              &
              metar_rel_hum_error,           &
              metar_temp_error,              &
              metar_wind_error

dewpoint_obs_err_mod.f90 is different to ncep/ecmwf_obs_err_mod.f90

[hkershaw-brown]

obs_utilities:

#129

should these get built when you build model/work or observation_converter/work (or both). These utilities are hidden at the moment.

Edit #129 was fixed in pull request #348. These utilities are all in DART/assimilation_code/programs so they can be built with any model/converter.