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.

[alexpearce]

There are lots of functors and most of them require at least a little explanation on how they're computed (DIRA, DOCA, IPCHI2, FDCHI2, M vs. MM, ...). Having to explain what each one does at the point when we first meet them doesn't sound unreasonable.

However, it's true that we don't have a general over of how we go from raw detector information to reconstructed decays. We mostly just say 'do the reconstruction' and 'make combinations'. I don't know whether this sort of material fits the first analysis steps, or belongs more in the (proposed) Physicskit.

[apuignav]

I think we want to keep the two things as separate as possible.

Going in the same direction as my comments in some issues, we risk overcomplicating things, and then it's very easy to miss the point.

My proposal (should I open a separate issue for discussing this?) would be to either have:

• Lessons split by easy and advanced. In the advanced part we could develop more some details.
• Have "Advanced" callouts that expand a bit what we're discussing.

In this round, while a lot of the additions have greatly improved the quality of the lessons, they have also made them more complicated. I think this goes a bit against the SWC philosophy of having short and concise lessons, and makes it more difficult for new students to follow.