ParentIdefId and Xform properties to InstanceReference

Added two public properties to InstanceReferenceGeometry class in
opennurbs_instance.cs: ParentIdefId and Xform.
Correction to XForm in Instances

As per Steve's comment, removed the redundant m_cached_transform and
simplified getter for XForm
Merge pull request #149 from mcneel/Additions-to-InstanceRef

ParentIdefId and Xform properties to InstanceReference