Remove leaking of tracks and replace it by construction from steps

[SeverinDiederichs]

This PR is a follow up to #539 and should not be reviewed before that one is merged.

This PR extends the idea to use the GPUHits to create the tracks for nuclear reactions to use it for all leaked tracks.
This has the following consequences:

• all leaked tracks related machinery is removed, drastically simplifying the state machine, the asynchronous GPU transport loop, the interfaces
• the deferred nuclear steps and leaked tracks can now be treated as the same again and the sorting is simplified

Regarding performance:

• sending back leaked tracks is cheaper than steps. Therefore, this new implementation can be a) cheaper if a step + a track were sent back, b) more expensive before only a track was sent back, as the step is bigger.
• simplifying the state machine and removing synchronizations should not really make a difference, as the leaks were usually not the problem.

To Do:

• run test in full Athena to see for the performance

It was verified that this PR

• Changes physics results
• Does not change physics results. Note: this PR does not change the physics results, Fix handling of MC truth for tracks undergoing nuclear reactions #539 did, so as soon as that one is merged and CI can be re-run, this should be flagged as no physics change.

[phsft-bot]

Can one of the admins verify this patch?