`sheath_boundary_simple` should warn/error if run with multiple species

[ZedThree]

sheath_boundary_simple is only designed for single species and is missing some factors that appear in the full expressions. Unfortunately, it seems that we don't know how many species we have when we construct the component, and only have this information the first time we call transform.

So, questions:

1. should we warn or error in sheath_boundary_simple if run with multiple species?
2. if so, is it fine to do so in transform? While doing so in the constructor would be best, this might require a larger refactor of Component and the scheduler
3. if not, should we detect whether we're running single- or multi-species and include the correct terms as appropriate?

[mikekryjak]

I think 1 and 2 are fine. @bendudson ?