Add Plane.freeze() method

[andykee]

It is very inefficient to repeatedly calculate dynamic plane attributes (think phase for a segmented system). It would be nice if there were a freeze() method that returns a new "frozen" Plane with all static attributes derived from the parent plane.

This method should operate on a standard set of attributes (probably defined in a private list) and also accept *args so that additional attributes can be added on the fly as needed.