Draft: Add two-track CEP line builder
Adds TwoTrackCEPLine, which selects low-multiplicity events (defined by the number of VELO or long tracks) with a 2-track candidate in a given mass range. Also adds a dimuon line to the default HLT1 configuration that selects J/psi and psi(2S) as an example.
The TwoTrackCEPLine can also be configured to handle cases such as CEP of D^0+\overline{D}^0, where we trigger on a single two-body D^0 (or \overline{D}^0) decay in a low-multiplicity event.
TODO: By default, vertices in events with no PV are only created if both tracks are identified as leptons. As a result, CEP lines will only give sensible results for dileptons for now. TwoTrackCEPLine makes the dilepton requirement optional for future use.