Add a lesson on 'how do we measure variables'

[vlisovsk]

While we teach the data flow in LHCb, it is not explained how do we measure the momentum, perform a vertex fit or define the primary vertex. I had to spend a significant amount of time explaining this during the LoKi functors lesson (e.g. to explain the difference between M and MM), following by questions of students. It would be convenient to have a little explanation on this before we move to exploring the actual physics variables.