Implementing LinearConstantStressQuadrilateralStiffnessMatrixBuilder

LinearConstantStressQuadrilateralStiffnessMatrixBuilder has been changed
to match LinearConstantStrainTriangleStiffnessMatrixBuilder
implementation.
StrainDisplacementMatrix and ShapeFunctionVector methods of abstract
ElementStiffnessMatrixBuilder now expect IFiniteElementNode as an
argument.  Have updated all inheriting classes.
Added ModelType.Membrane2D to enum

Added ModelType.Membrane2D to the ModelType enum.
Included behaviour for Membrane2D in necessary methods which refer to
ModelType.
Triangle membrane test uses ModelType.Membrane2D
Test for Quad Membrane types