Optimise the generate_atm function

Reference issue

Fix #986 (closed)

Description

Supersedes MR !179 (closed) by relying on TelemacFile instead of the deprecated Selafin class. This new implementation provides the same performances as that of @saillour.thomas, yet is simpler. Since it uses TelemacFile, generate_atm should now also work with MED files when the HERMES Python wrapping is built.

I've also taken the opportunity to correct a small issue by renaming the incorrectly named ll2utm option to utm2ll.

Checklist

All Merge Requests:

• Update NEWS.txt to describe your changes.
• Run compile_telemac.py --check-code to check FORTRAN coding conventions.
• Run pylint to check Python coding conventions.
• If you added new files, run compile_telemac.py --clean --rescan and commit the updated cmdf file(s) accordingly.
• Run compile_telemac.py --clean for both normal and debug configurations.
• Run validate_telemac.py for both normal and debug configurations.
• Run validate_telemac.py --notebook for both normal and debug configurations.
• Run doc_telemac.py if there are any modifications in the documentation.
• Run damocles.py --eficas if there are any modifications in the dictionaries.

Additionnaly, 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.