Allow for Massive Missing Particles in TupleToolCorrectedMass Merge to run2-patches

Revision of previous request now working from correct branch: run-2 patches. Also adjusted is contribution to error calculation. Info on what the change to the TupleTool does:

Adding a few lines to TupleToolCorrectedMass so that missing particles with mass can be included in mass reconstructions. This is for an analysis of Lb -> LcLc~n where we are considering the neutron a missing particle. Unless explicitly stated the mass of the invisible particle is by default 0 and the TupleTool acts as it was previously, so users do not have to specify m_invisible = 0.

Adding lines like this in the options file will result in non 0 invisible mass.

dtt.addTupleTool("TupleToolCorrectedMass/M_n")
dtt.M_n.MassInvisible = 939.56542052 * MeV