Model Stability Checks and Improvements to Meshes

• PyNite now checks for nodal instabilities when analyzing a model. If nodal instabilities are found, PyNite will output the unstable nodes and directions to the console, and will throw an exception.
• Added a method called rename to the FEModel3D class for quickly renaming all the nodes and elements in the model in sequential order.
• Added a last_node and last_element attribute to the mesh class. These methods can be used to get the name of the last node or element in a mesh.
• Improved the reliability of the add_mesh method. It now can handle adding meshes containing node and element names already defined in the model. It automatically resolves the duplicate names.