Fix bug in track refitting to support mixed track types (downstream + long)

• Originally, the SelectTracksFromParticles used the key of the track to check if they were already in the container. This does not really work in case you have mixed input containers for the tracks. Instead, we use now the SharedContainer's index() function to see if the object itself is in there or not.

As a bonus, this actually simplifies the code...

• Fixes the RecombineDecayTrees to no longer have a memory leak in case there is a particle without a proper relation.