Philippe 29/01/2018: FIND_FIELD_ID_FROM_MNHNAME: add optional argument ONOWARNING to remove warning messages when they are normal (debug level)

 Conflicts:
	src/MNH/diag.f90

+ correction for DMFCONV attributes

Philippe 25/01/2018: IO: _FillValue cannot be modified (or re-written) if some data has already been written

* add latitude(_x) and longitude(_x) fields in NC4 files
* set the coordinates attributes for fields
* commented out CSTDNAME for LAT and LON fields to not have 2 times the same standard_name
* corrected NGRID for DX/YHAT fields
* modify condition to detect if program is PREP_SURFEX

Philippe 23/01/2018: IO: add support for Arakawa uvw point in NC4 files (probably not used but set for completeness and future use)
+ small improvement for uw and vw points (more adequate choice of coordinates)

'origin/MNH-53-branch ::Philippe 16/01/2018: bug correction in GPS_ZENITH(_GRID)' into MNH-54-branch

+ correct c_grid_dynamic_range attribute
+ add COMODO-1.4 to Conventions attribute