"Disappearing" track name change

Following discussions with @elsing @npetters @mdanning @goetz @sroe it was felt that the naming of the PixelPrdAssociation tracklets is misleading (since the algorithms filling these could easily be mistaken for an algorithm running PrepRawData Association to tracks), and should be changed. A more physics-oriented naming is applied here, reflecting the primary use of these tracks/track particles.

Tracking output looks consistent after the changes, aside of course from the change in the container name.