Rendering via `Pyvista` and Other Improvements

• Added rendering via Pyvista. This greatly simplified the rendering code and provided a fresh look to the rendereings. Renderings in jupyter are now interactive. Global axes are also now shown in rendereings. To use Pyvista instead of VTK, use the new Rendering library rather than the old Visualization library. Rendering via VTK directly is still available.
• Bug fix for member self-weight. The program was throwing exceptions instead of calculating member self-weight. Added a unit test to help prevent this issue from occuring again as code changes.
• Refactored material to be material_name in the code. The prior naming convention caused confusion which led to the self-weight bug.