WIP: Add UT hit index to output of PrVeloUT

Add UT hit index to output of PrVeloUT. This still needs to be adapted to the final things in LHCb!2561 (merged).

Some other cleanups

Needs: LHCb!2561 (merged) @peilian

Changes include:

• Addition of UT hit indices
• Cleanup of the code
• Remove of unnecessary structures and properties