Performance Improvements

• Sections have been introduced to allow for member stresses to be tracked by the program during nonlinear analysis in the future. This opens the door for other useful features down the line too. Use of Sections is optional, and will only required for pushover analysis when that feature is implemented. You do not need to use this feature yet.
• P-Delta analysis code has been greatly simplified. Performance has also been improved, as redundant iterations are no longer being performed.
• Removed the tol parameter as it is unecessary when using a geometric stiffness matrix instead of an iterative procedure.
• Better documentation for P-Delta analysis.
• Corrections to unit tests that weren't working properly. Added another AISC Benchmark unit test for P-Delta analysis. This should help safeguard the program against some types of bugs being introduced going forward.
• Bug fix for multiple support springs at a single node. When calculating reactions, the program was only considering the effects of one spring at the node, whichever came first in this list: DX, DY, DZ, RX, RY, RZ. This only affected reaction calculations, and has been remedied.
• Bug fix for mesh max/min results. The program was throwing exceptions when the user requested results from the mesh rather than the elements themselves.
• Work has begun on nonlinear pushover analysis. This has proven to be a challenging feature to implement, and still has a long way to go. However, in the process of working on pushover analysis several inneficiencies in the analysis code were identified and fixed. This update includes those changes, and brings the "work-in-progress" pushover code into the main branch of the project. The incomplete pushover branch was brought into to the main branch because critical parts of the analysis code were diverging from the main branch, making it harder to accept pull requests from other users.