Use model bounding box to accelerate plotting

[pshriwise]

Description

Generating plots with views that extend beyond the model can be costly because the plot algorithm must rule out all cells before determining that the pixel center doesn't reside in the model. This is most noticeable in the plotter.

One way to accelerate this is to make use of any finite dimensions of the model bounding box.

Alternatives

Methods for calculation of cell/universe bounding boxes more generally would allow find cell operations to naturally incorporate a bounding box check, thus accelerating plot operations in essentially the same manner.

Compatibility

This would be an entirely internal change to the code.