use only children momentum to recalculate MASSWITHHYPOTHESES

When calculating the mass with alternative mass hypotheses for the children, MASSWITHHYPOTHESES uses:

• a) energy calculated with children momentum
• b) magnitude of the parent particle momentum. This results in an incoherent combination of momentum values if the parent particle has been obtained by a combination/vertex fit and so its momentum has been recomputed. With this MR, in b) the parent particle momentum is replaced by the sum of the children momentum, as suggested by @graven .

This addresses #307 (closed)

To be tested with DaVinci!1208 (merged)