Philippe 15/12/2016: IO:

* added TPFILE argument to IO_WRITE_FIELD_NC4
* added optional KZFILE argument to IO_WRITE_FIELD_NC4_
* provide the correct file to IO_WRITE_FIELD_NC4 if z-splitted
  file in IO_WRITE_FIELD_BYFIELD_N0
* added TFILE_IOZ array of pointer to TOUTBAK type to point
  to z-splitted files
* manage z-splitted files in IO_FILE_OPEN_ll and IO_FILE_CLOSE_ll
* manage z-splitted files in IO_PREPARE_BAKOUT_STRUCT
* created IO_FILE_FIND_BYNAME subroutine
* merged linked lists concerning output and backup files