Allow FrameSense to see and use third-party operators that live outside its code base

[geoffroy-noel-ddh]

The aim is to enable a more open architecture where operators can be developed and stored individually outside Framesense repo.

We could let the user point to additional 'operators' paths outside FrameSense code base.

Or use plugin_operator/plugout_operator operators to symlink external operators into FrameSense ./operators folder.