Fix TypeError issue with extruded property for Subsets

Simplify extruded property (set a flag to False on Sets, True on
ExtrudedSets) and make Subsets inherit from ExtrudedSets.  This latter
change fixes strange TypeError issues when iterating over Subsets.
Merge pull request #325 from OP2/extruded_property

Fix TypeError issue with extruded property for Subsets

Simplify extruded property (set a flag to False on Sets, True on
ExtrudedSets) and make Subsets inherit from ExtrudedSets. This latter
change fixes strange TypeError issues when iterating over Subsets.