lib.plot: Remove the deep copy in PolyPoints.points property

[dougthor42]

(This issue was created because PR #117 could not be finished in its entirety)

There is a deep copy of the _points attribute in the lib.plot.polyobjects.PolyPoints.points property that might be able to be removed.

Investigation needs to be done to see exactly why the deep copy is needed. From there, work can be done to figure out how to remove it.

Removing the deep copy should help with RAM usage for high-point-count plots.