Functor for error on corrected mass / pT of daughter relative to mother / p (E) in rest frame of mother, with approximation

• Functor for error on corrected mass
• Functor to get pT of daughter relative to mother momentum (e.g. for B -> rho mu nu, the pT of the rho wrt to the B flight direction, using the rho+mu vertex (and not just the rho one))
• Functor to calculate the energy or momentum of a daughter particle in the rest frame of the mother, whose momentum is given by the rest-frame approximation.

@vekirseb