Template VPTrackMonitor to output histograms

Template VPTrackMonitor to output histograms or tuples depending on purpose. Histograms are needed to display histograms on Monet for the online Velo monitoring as tuples shouldn't be used as input for Monet.

@graven @decianm @bimitres @sborghi @mhilton @hcroft

I've checked that the tuples from VPTrackMonitor are not broken by these changes:

• run checkVPTrackMonitor.py from the main and the new branch
• plot output using plot.py
• see identical distributions plots.zip

This is ready to be merged. Eventually and in a future step, we will add a few histograms using the new class from Draft MR !2818 (merged)