Define comparator decorator for LHCb::ParticleProperty objects.
Closes #66 (closed).
This doesn't fix what I'd consider to be the 'underlying' cause, that the operator< method isn't being used in the Python bindings for LHCb::ParticleProperty, but that's a deeper issue that'll take longer to figure out. Fixing that problem would be great, because then one wouldn't need to import PartProp.decorators to pick up this fix.