MAINT: Move single cycle/diagram plotting code into new, public plotting function

[nawtrey]

As discussed in 6. of this comment, the single-diagram/cycle cases of draw_cycles() and draw_diagrams() probably belong elsewhere, potentially in a separate, public function. It seems a little awkward to input a single diagram/cycle into plural-named functions anyways, although a general plotting function might want to handle this case.

This will have to be resolved after #47, and likely after #48.

[nawtrey]

I don't think this is necessary. I think it would be way more awkward to have multiple functions to handle 1 vs. 2+ diagrams. With the new KineticModel object, I think it will be easy to build methods for easy plotting of diagrams. I'll open a separate issue for that when the time comes, so closing this.