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. 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.