AROME fix for the c5b54ddd commit

Creation of the temporary 'externals' directory for AROME.
This directory will contain the AROME specific version of
source codes that is not part of the PHYEX package but is
needed by the package.

Main mods:
- XRTM17, LOCEAN and ocean related constants added
- AROME specific version of modd_precision, modd_dynn created
- creation of a message system for AROME: modd_io and mode_msg

Other mods:
- mv budget in externals
- creation of the docs directory (containing working documents)
- creation of src/arome/gmkpack_ignored_files; the file contains
  the list of the source codes which must be ignored by gmkpack
  but was present in the main pack. This file is necessary while
  the ecboundle mechanism is not used.