Undo creates unwanted objects

[zbynek]

Run probability calculator, close it, press undo: normal curve and points appear.
Similarly with single variable statistics.
Two-variable and mutivarible analysis and function inspector seem to be OK.

[george]

Undo problems were solved by changing the way in which GeoElements are created in these classes. Instead of sending strings to the algebra processor the geos are now created by directly constructing the needed algos. This allows the objects to be kept out of the XML, preventing undo problems.