modify find_duplicates_sharedhits

[leonardogiannini]

this PR modifies find_duplicates_sharedhits used in pixelless and tobtec to fdollow a logic similar to cmssw

1. maps hits to tracks
2. find for each tracks the ones the share hits with it and how many

this allows to avoid nested track-track loops

the timing is reduced by a factor from ~2 to 4, while the physics is not changed i.e. same number of output tracks

• new vs old time for the function (10 evts)
  

• speed up plot (10 evts)
  

[mmasciov]

Closing, as superseded by cms-sw/cmssw#37122.