Fix bug with ExtractGeometry filter

The ExtractGeometry filter was outputing datasets containing CellSetPermutation as the representation for the cells. Although this is technically correct and a very fast implementation, it is essentially useless. The problem is that any downstream processing will have to know that the data has a CellSetPermutation. None do (because the permutation can be on any other cell set type, which creates an explosion of possible cell types).

Like was done with Threshold a while ago, this problem is fixed by deep copying the result into a CellSetExplicit. This behavior is consistent with VTK.

Backport: release