[PWGEM] New Dielectron analysis type: Bootstrapping for v2.

[omassen]

Introducing a new analysis type for performing poisson bootstrapping for the dielectron v2 analysis. This includes:

• changes to the EventHistorgams.h to introduce new histograms per bootstrapping sample and a method of filling them
• Small change to PairUtils.h to be able to choose the new analysis type.
• In Dilepton.h implementing the new analysis. Existing code could for the main part remain untouched. The only change to the existing code is the introduction of a new argument for the FillPairInfo function, needed to parse the bootstrap weights.