Add LoKi MTDOCACHI2 Functor to ThOr

I am working on the B-->K+pi0 line upgrade to run 3, and I've discovered a LoKi functor with no ThOr equivalent, called MTDOCACHI2. This functor is described [here](https://cds.cern.ch/record/2714062/files/LHCb-ANA-2020-022.pdf:

A trajectory is made from the primary vertex with the smallest impact parameter χ2 to the daughter K+ along the direction of the reconstructed B+ candidate momentum. The MT-DOCA is the distance of closest approach (DOCA) between the daughter K+ and this trajectory.