Simple over-dimension check for __contains__.

And small edits to tests to accommodate.
Allow definition of implicit and inverse equations.

Includes simple checks in __init__, but does not yet otherwise make use of
these equations.
Change __contains__ to contains.

Turns out __contains__ will implicitly convert any return value to a boolean.
This will certainly not do when I plan to have it return conditionals soon.
Extra check on init with implicit: parent must have inverse.
Support implicit and inverse in in_ancestor.

Converts equations into ancestor space along with coords.