Philippe 27/07/2017: IO:

* added IO_FILE_ADD2LIST subroutine and use it
* IO_FILE_OPEN now requires the file to be in the file list
* IO_FILE_FIND_BYNAME: added optional argument useful when checking
  that a file does not yet exist
  + bug correction when filelist is empty
* added IO_FILE_PRINT_LIST subroutine (only for debugging)
* added TPTR2FILE structure to allow construction of arrays of
  pointers to files
* use IO_READ_FIELD for some 3D float arrays (in read_field.f90)
* OPEN_ll: added UPDATE_METADATA internal subroutines to treat file metadata