Add fortran and python API's for triggering the output of telemac2d complete restart files.

This Merge Request closes #823 (closed)
It introduces the API for triggering the output of complete restarts for telemac2d as recently introduced in fortran sources by @chi-tuan.pham
Notice that the main switches for enabling the RESTART_MODE and setting the name and format of the restart file have to be correctly positioned in the .cas file.
Even if it would be comfortable to handle these settings (and the companions for the COMPUTATION CONTINUED and PREVIOUS FILE name and format entries), currently the safety checks on the available files and the task preparation for PARTEL and GRETEL are only based on the contents of the .cas steering file.
The validation example t2d_restart.py shows how to modify on the fly the steering file of the gouttedo case, via the TelemacCas python class.