Add evtgen keywords to metadata

With recent-enough EvtGen versions, physics keywords are stored in the TagInfo metadata. This MR allows them to be read with read_metadata() in PyUtils and adds them to config flags as Input.Keywords.

I imagine these would be generally useful but in my specific case these will be used to determine whether or not to run the heavy-flavour classifier defined in DerivationFrameworkMCTruth.

I am not an expert in Athena's metadata processing so it's possible there are other changes that need to be made to make this consistent.