Philippe 27/04/2017: IO: added writing of 3D array of integers

* added IO_WRITE_FIELD_BYNAME_N3 and IO_WRITE_FIELD_BYFIELD_N3 subroutines
  to IO_WRITE_FIELD procedure
* added IO_WRITE_FIELD_LFI_N3 subroutine to IO_WRITE_FIELD_LFI procedure
* added IO_WRITE_FIELD_NC4_N3 subroutine to IO_WRITE_FIELD_NC4 procedure
* possible to write 3D integer values in output files
* added ALLOCBUFFER_N3 to ALLOCBUFFER_ll procedure
* added GATHERXX_N3 to GATHER_XXFIELD procedure
* added GATHERXY_N3 to GATHER_XYFIELD procedure