Philippe 06/04/2017: IO: added writing of 4D array of real

* added IO_WRITE_FIELD_BYNAME_X4 and IO_WRITE_FIELD_BYFIELD_X4 subroutines
  to IO_WRITE_FIELD procedure
* added IO_WRITE_FIELD_LFI_X4 subroutine to IO_WRITE_FIELD_LFI procedure
* added IO_WRITE_FIELD_NC4_X4 subroutine to IO_WRITE_FIELD_NC4 procedure