Add more sanity checking of constraints

At the moment Zool isn't very intelligent at constraints. For example, it's possible to add a circular constraint where a `width` constraint can point to itself. To close this issue we want Zool to be able to examine `PlotElements` and their constraints at runtime to see if there are any *obvious* issues.

Start by locating the PlotElements implementation and how constraints are represented and evaluated at runtime. Trace how a constraint is added and checked, using the self-referential width example as the initial case. Done means Zool can identify obvious invalid or circular constraints when examining PlotElements and their constraints.

Written by the indexing model from the issue text.