Removed mutables from PrAddUTHitsTool

I removed all mutables from PrAddUTHitsTool. There might be a very small loss in timing, as one variable is now calculated per track instead of per event.

Validation started with lhcb-future-clang#38

Validation started with lhcb-future#245

@sponce I don't know if you get a message for these automatically, so I'll add you explicitely.

I get all messages automatically ! I'll merge this as soon as the nightlies are ok. Sadly today they were failing in LHCb do to something unrelated, so let's see tomorrow

Validation started with lhcb-future#246

Validation started with lhcb-future-clang#39

Validation started with lhcb-future#246

Mentioned in commit a744307c

Status changed to merged