Added new variables and tests to study and test decays with FSR

Summary:
Added new variables to testing module.

Added new tests for FSR including tau decays.

Implemented const correctness for getP4LabBeforeFSR() function.

Use momentum before FSR to rank charged particles for FSR variables.

Updated History.md

This MR is associated with !15 and T229

Test Plan: Run new tests and check that the output is OK.

Reviewers: kreps, tlatham, jback

Reviewed By: tlatham

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D111