Add functor to access decaytreefitted particles

Implementing https://gitlab.cern.ch/lhcb-dpa/project/-/issues/117 together with Phys!970 (merged) and DaVinci!569 (merged)

Not complete and not functional, but already in an MR so that anyone can see and help (!).

We did a design session at the whiteboard with @graven. We decided to factorise the problem into two steps:

• Make a functor that accesses the related particle and returns pt
• Replace pt by a functor PT, or any functor you like.