15/5/2018 : (S. Riette) Add ZPABST to keep pressure constant during timestep + (J.Colin) Add VISCOSITY

* IO_READ_CHECK_FIELD_LFI: reduce error level to warning if unexpected grid for scalar variable
* IO_FILE_OPEN_ll: missing NDIMS for MASDEV
* INI_MODEL_n: associate TINIFILEPGD to TFILE_DUMMY if necessary
* WRITE_BUDGET: set NGRID=1 for 6D MASK variables
* WRITE_DIACHRO: reverted name of field to old behavior (kept improved comment) (patch was lost in previous merge)
* PREPARE_METADATA_WRITE_SURF: keep TPFIELD fieldvalues when in conflict with passed values

Philippe 03/05/2018: IO: check if NAM_BACKUP namelist is present and that the list of valid backups is not empty

 Conflicts:
	src/MNH/spawn_field2.f90
	src/MNH/spawn_grid2.f90
	src/MNH/spawn_pressure2.f90
	src/MNH/spawn_surf2_rain.f90