Beginning implementation of LinearConstantStressQuadrilateral
Merge branch 'master' of https://github.com/iainsproat/SharpFE
Added example for 1D beam portal frame

Example for 1D beam portal frame has been added.  This works, unlike the
same example for the 3D beam.
Further work-in-progress work for the Quad4 element has been included.
The test fails.