[gaia] RESTART MODE for GAIA like in TELEMAC-2D and TELEMAC-3D

First part.
Partially closes #756 and #899.

Major improvements:
- automatic saving of variables in the RESTART FILE necessary to continue a
GAIA computation at the wished records and not every graphic printout,
- as necessary variables are not saved in the RESULT FILE anymore
(in particular at every graphic printout), this last file is smaller in size
(e.g. with CO sediments),
- no loss of accuracy when continuing a computation.

New keywords:
- RECORD NUMBER FOR RESTART,
- RESTART MODE,
- RESTART FILE,
- RESTART FILE FORMAT,
- RESTART FILE PRINTOUT PERIOD,
- RECORD NUMBER IN RESTART FILE.

Examples in:
- mud_conservation-t2d,
- yen-t2d (multi 1),
- littoral-t2d-tom (triple coupling).

Updated GAIA reference files due to less stored variables (CO type sediments).

NB: some continuations of computation are not currently perfect (loss of
accuracy) and need to be investigated (coupling with TELEMAC-3D, CVSM...).