Add z dimension to variable mesh

[matthewturk]

Adding a 'z' dimension to the variable mesh would enable passing rectangular structs (or cutting planes) to the code and allow for rapid switching between layers.

The means of doing this seems straightforward enough -- just add another variable (the value at which to slice, for instance) but I'm not quite experienced enough in rust to know off-hand the best way to implement it. Ideally it would add on a new trait to a different kind of variable mesh pixel object, and then implement the deposition slightly differently depending on that, but I'm not completely sure.