Add corrected mass error functor
Adds a functor for corrected mass error.
Addresses part of Moore#476.
Not an exact copy of the LoKi functor - I have attempted to condense the algebra.
To be tested with: DaVinci!763 (merged) Moore!1922 (merged)
Requires LHCb!3882 (merged)
Edited by Miguel Ramos Pernas