Trk::Track : Hold the TrackStates as ptr rather than value. Otherwise we lose the DV polymorphic behaviour

Trk::Track :

Hold the TrackStates as ptr rather than value. Otherwise we lose (splice) the polymorphic behaviour of the DataVector.

e.g we "effectively" we want

struct foo {
 std::unique_ptr<Base> m_bar;
};

rather than

struct foo {
 Base m_bar;
};

So at to able to recover Derived.

The DataVector although is a kind of a vector also has this polymorphic behaviour.

Actually this was tried in smaller scale (when we wanted to have custom allocators here) !46853 (diffs) (let me ping @ssnyder but in principle I think we should have kept that and double down as breaking the polymorphic behaviour of the DV creates some other issues also)

The main changes

• We hold a unique_ptr in the Track.h
• We expand the relevant test to cover things that this now allows us to do.
• We change the clients to pass unique_ptr . With respect to the 1st older try the unique_ptr is more visible i.e in the one in the past there was one unique_ptr created ,anyhow, via a copy of the value.