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Encapsulate Part::Geometry data member copy process in a single function (copyNonTag).
Part::Geometry derived classes need not know the specifics of the data member of Part::Geometry.
Change the delegation of the copy of extensions from clone() to copyNonTag. Because clone() relies on
copy() and now copy() of Part::Geometry data member relies on copyNonTag(), there is no actual change
to the clone() functionality, but the copy() funcionality gains the copy of the extensions via copyNonTag().
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