Add two notebooks related to the use of postel for GAIA calculations

This Merge Request concerns the addition of 2 new notebooks on post-treatment in Gaia thanks to @coraline.bel. One notebook concerns calculation of Rouse and Shields, the other one concerns the calculation of mass balance This last one should be modified in the future.

Reference issues

#1121 and #1122.

Description

Checklist

All Merge Requests:

• Update NEWS.txt to describe your changes.
• Run build_telemac.py --check to check Fortran coding conventions.
• Run pylint to check Python coding conventions.
• Run build_telemac.py --rebuild for both Release and Debug configurations.
• Run validate_telemac.py for both Release and Debug builds.
• Run validate_telemac.py --notebook for both Release and Debug builds.
• Run doc_telemac.py if there are any modifications in the documentation.
• Run damocles.py --eficas if there are any modifications in the dictionaries.

In addition, for a new feature:

• Check that your feature works in both serial and parallel modes.
• Add at least one test case to check the functionality (with documentation, graphics and VnV script)
• Update the documentation for the module in which your feature will be available.