(cherry picked from commit e83d72ab)

+ more efficient use of grid variables

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

* added IO_READ_FIELD_BYNAME_X0/X2/T0 and IO_READ_FIELD_BYFIELD_X0/X2/T0 subroutines
  to IO_READ_FIELD procedure
* added IO_READ_FIELD_LFI_X0/X2/T0 subroutines to IO_READ_FIELD_LFI procedure
* added IO_READ_FIELD_NC4_X0/X2/T0 subroutines to IO_READ_FIELD_NC4 procedure
* use IO_READ_FIELD in SET_GRID

Philippe 18/11/2016: bug correction: call IO_PREPARE_BAKOUT_STRUCT only if program is MESONH + set default values fro NBAK/OUT_NUMB to 0

+ separate outputs and backups lists

Philippe 08/11/2016: moved preparation of backup file structures to a new subroutine: IO_PREPARE_BAKOUT_STRUCT

Philippe 29/09/2016: added new fields (and renamed 2 of them) to TFILEDATA datatype and began to use them

Philippe 13/09/2016: added TFILEDATA datatype to describe the characteristics of the files and begins to use it
Plus : do not write NAM_FMOUT (due to some fields not allocated anymore)

Philippe 06/09/2016: added XBAK_TIME_FREQ and XBAK_TIME_FREQ_FIRST to choose backup frequency in terms of time

Philippe 05/09/2016: added NBAK_STEP_FREQ and NBAK_STEP_FREQ_FIRST to choose backup frequency in terms of timestep
(+ corrected a small bug if no backups at all)

* added fields NBAKID, NOUTID, CFILENAME and CDADFILENAME to TOUTBAK datatype + use them
* building of the generic output name was displaced before SET_GRID in ini_modeln

* Find dad file number in SET_GRID (added NOUTDAD variable in TOUTBAK datatype)
* Allocate XBAK_TIME to the right size

* added TOUTBAK datatype to manage the outputs/backups characteristics
* TOUTBAK replace NOUT_TIMES (removed)
* XBAK_TIME is now allocatable

* this is the beginning of the implementation of separated outputs and backups
* also, improved things by removing duplicated values and sorting them