Philippe 15/05/2018: IO: bug and problems corrections:

* 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