Minimal change to WFinder to remove neutrino reliance.

The WFinder needs a major rethink. For now, I have only removed the
reliance on enumerated neutrinos, and replaced it with the missing
momentum 4-vector. Any reference to neutrinos is now taken to be the total
missing momentum rather than the hardest neutrino.

Watch out:

• The particles() method returns no neutrinos.
• If there is more than one lepton in the event, no W will be found.