Thread safety cleanup in PhysicsToolsObjects

I first looked at this because reports of potential
thread safety problems from the static analyzer,
but a recent commit fixed those already. I added
a comment before the call to EvalPar as I spent
some time looking at it.

Also added a return statement in
PerformancePayloadFromTFormula::printFormula
when the index fails the range check instead
of trying to use it anyway.