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)

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

* use metric horizontal coordinates in the transformed plane when available
* write vertical coordinates (SLEVE not yet done)

Philippe 15/01/2018: IO: write horizontal dimensions in NC4 files: improvements + problem corrections

Philippe 12/01/2018: IO: add ZTOP field for model top (useful for mesh description in CF convention)

* HDF5 1.8.9 -> 1.8.20
* netCDF 4.1.3 -> 4.5.0
* netCDF-fortran 4.1.3 -> 4.4.4
* added libaec 0.3.2 (equivalent to szlib but without licence problems)

+ use them

* FD_ll removed several fields not used anymore
* removed GETFD_COMM function
* bug corrections: use NLFIFLU instead of NLU
* bug correction in CLOSE_ll: .NOT. GPARALLELIO => GPARALLELIO