Added rectangle functionality.  Corrected some minor documentation issues.  Added addition cell test files.
Reordered trapeziod & tetrahedron point numbering to be more consistent with cone, cylinder, arc, etc.
Implemented cell instances.  This required some refactoring of the Set cell hierarchy.  Definitions and RegularSets now inherit from SetsWithChildren, which implement sets having multiple children.  Instances are also sets - although they have only single children.