Document turn dynamics calculations, most importantly FTO

[mdingemanse]

We have a fairly well-considered form of FTO calculations that is a substantial python improvement of a python port of my original R code. The code is in conversation.py.

However, we don't have good documentation yet of how FTO is calculated and why.

Here is a lot of useful material, including a great flowchart by @bvreede.

[mdingemanse]

BTW this is especially important because choices made early (e.g. which tiers to import) directly impact FTO calculations. E.g. if you import an IFADV file and it has tiers for speech and for gaze, scikit-talk won't know which is which and FTO calculations will simply not be carried out because it's detected as four participants and we (sensibly) only compute FTO for clearly dyadic data.

A vignette or workflow that explains this would be good.

Also, flexible ways to recalculate FTO using custom settings, which I'll make another issue for.