Remove direct use of Sirius solver : first removal

[guilpier-code]

This PR intends to be a first work in order to remove the direct call of Sirius as a solver, regarding the optimization weekly problem only. We want to keep the use of Sirius, but only called through or-tools interface. The associated gopro ticket is this one. Here we intend to :

• remove the use of command line option --use-ortools
• rename current command line option --ortools-solver into --solver
• handle direct consequences of these changes.

So this PR does not intend to be thorough : these first changes leave remaining dead code to be removed and renaming to be done. This further changes will be done in a coming PR.

Remaining to be done :

• or-tools (even if we use Sirius behind it) does not print correctly MPS for named problems : optimization variables names are composed with time steps, set once for all (0, ..., 167) before running any year, and not updated at each week (we could expect step numbers to be inside [0, ..., 8759] depending on the week). So we left the code that print these correctly. We'll have to find a solution for this. We could leave the responsibility of printing these named MPS to Antares code (as it did when Sirius was run alone) for a moment
• We should remove call to Sirius alone in the legacy GUI (not long).
• Banish all direct calls to Sirius from Antares Simulator (calls to Sirius are done in hydro part, adequacy patch and annual quadratic post-treatments of flows) does not seem to be suitable and urgent for now.
• What else ?