Functor to calculate the invariant mass of a resonance (or any combination of particles), including one missing particle, using the mother mass and flight direction as a constraint.
e.g. M -> A + R, R -> X + B1 + B2 + ..., with M=mother, R=resonance, X=missing.
Can also be used to calculate q2 for semileptonic decays.
@vlisovsk @chefdevi @masmith @mrotondo
