New ttH topology for truth content in DAOD_PHYS

Aim

This merge request introduces a new truth-level topology which aims to retrieve information on (truth) Higgs bosons produced with a ttbar pair (ttH). This should run on both DAOD_PHYS and earlier samples (though it has only been tested with a DAOD_PHYS sample).

Method

I used the ttbar and Thq topologies as a reference, the former being the only topology that (currently) supports reading from DAOD_PHYS samples in release 22. After implementing the necessary changes, the code was tested on a ttH MC sample, and compared to the ttbar topology. Distributions produced from both are comparable, though a deficit is seen in the ttH topology's ttbar/t/tbar/etc. distributions due to the requirement of a truth Higgs boson being present (line 79 of the new CalcTthPartonHistory.cxx file).

Note on distributions: I would think, perhaps naively, that a ttH signal sample would contain a truth Higgs in all events.