[WIP] Refactor user interface of JRhom algorithm (previously multi-J)

[EZoni]

Implement what was discussed during the hackathon and summarized in #5229 (comment).

If accepted, this PR should replace #5229, which addressed only renaming and has many merge conflicts.

The previous input parameters

• warpx.do_multi_J
• warpx.do_multi_J_n_depositions
• psatd.rho_in_time
• psatd.J_in_time

are replaced with the one new input parameter psatd.JRhom, a string such as CC1, CL2, etc., which encodes

• the time dependency of J through the first character;
• the time dependency of rho through the second character;
• the number of subintervals through the last integer.

This is aligned with the conventions used in the paper where we published the algorithm.

To-do:

• Rediscuss new user interface
• Improve parser of new user input psatd.JRhom
• Complete refactoring throughout the code base
• Add backward compatibility error messages
• Update standard output messages (assert, abort, warning, error, ...)
• Update documentation