Major bug fix for quadrilaterals & other improvements

• Major bug fix for quadrilaterals. In-plane stiffnesses were 1/4 of what they should have been.
• Added more unit testing for plates and quadrilaterals.
• Fixed a bug where cylindrical quad meshes didn't calculate the number of quads that fit in the circumference correctly. This bug only affected models where the number was not explicitly specified.
• Added plate/quad surface load validation to ensure loads are not added to ficticious plates/quads.
• Improved the remove_duplicate_nodes method. It wasn't working properly for springs. It also now returns a list of the names of the nodes that were removed. Also added a docstring for this method.
• Added the orphaned_nodes method that checks the model for orphaned nodes and returns a list of the names of orphaned nodes.