Create flowcharts for jointcal

There are a number of steps that have to happen before jointcal's fitter can even compute a chi2. We need a flow chart documenting those steps, if for no other reason than to make it easier for me to write tests that exercise different parts of that flow.

We also need a flowchart showing the internal algorithm of the fitter.

There is some discussion of these steps in sections 5 and 6 of DMTN-036, but we should flesh them out and diagram them.