SIMD field for flags / Adapt MuonPID for it

• Introduce SIMD field flags_v and the corresponding container, to act as a bit-field using and int.
• Adapt LHCb::Event::v2::Muon::PIDs to use the correct access to the flags, remove unnecessary old fields.
• Adapt tests for it.

Needed for Rec!2785 (merged)

@rvazquez @cmarinbe @ahennequ