allow control of *Resistance and FractionalFlow

[visr]

Fix #718
Fix #722

Not sure if this works yet or if the validation is just too strict. Before merging this should be covered by a test model.

[visr]

Copying commit message a52d490 for visibility:

Fix FractionalFlow sum to 1 validation with control states

The initial fractions come from whatever happens to be the last control state per node, and therefore can trigger false positive validation errors.

Now we validate this per control state. To make that work I had to add the missing control state to the control_mapping. For simplicity I coalesce a missing control_state to the empty string. I wonder if that is the right approach though, since for e.g. name we default to the empty string rather than support missing. We should probably settle on either for all string columns.

@evetion what do you recommend for missing / default string columns; missing or ""?

[visr]

I made #777 for extra test coverage, and folded #772 into here since it has related control fixes that needed this branch to test.

[SouthEndMusic]

Looks good (:

[evetion]

@evetion what do you recommend for missing / default string columns; missing or ""?

For naming things, so which not influence the numerics, I'm fine with "". Otherwise I like the explicit handling of missing with coalesce.