Updating tau rec tools to do less in beginEvent

BuildTruthTaus was previously using beginEvent as an opportunity to look at metadata -- to do a sort of late configuration on the fly. That isn't an idea way to do things here, so changing to use a simple function and a member variable cache, which is called in any of the applicable functions when the value is actually needed.

This solves a number of errors when running derivations in athenaMP.