MultiTrajectory buffer default constructible

[tboldagh]

While integrating CKF in Athena it was found that MultiTrajectory (Athena version) is also used as a temporary buffer in CKF:

acts/Core/include/Acts/TrackFinding/CombinatorialKalmanFilter.hpp

Line 1335 in fd8e801

auto stateBuffer = std::make_shared<traj_t>();

That would require either Athena version of MultiTrajecotry to become default constructible (i.e. properly own backends) or Acts::VectorMultiTrajectory is used as buffer. (probably the former is easier to test).

[tboldagh]

Thinking a bit about it ... what is realy needed here is "virtual constructor" pattern or kind of clone pattern.
For that a method: createTemporary() could be added to MTJ maybe? This way backends can decide on best implementation.

[paulgessinger]

Honestly, I really think the simplest thing to do is just have the CKF always use the transient backends, since it explicitly copies out the track states we intend to keep. This buffer is purely transient anyway, and I don't think there would be any performance benefit of actually using matching backends.

I'll try to change this when I'm back in the office.

[tboldagh]

The Athena backend can still decide to hand out Acts one in "createTemporary()"? Or we enter the "virtual" methods this way?

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