Nonlinear Analysis Improvements

• Added the ability to apply loads in steps in the FEModel3D.analyze method via the new num_steps keyword argument. num_steps defaults to 1, but if a higher number is used the load will be split into the specified number of steps and applied incrementally. This can be very helpful when dealing with complex tension/compression-only scenarios that otherwise may have had trouble converging. Consider using 10 to 20 load steps when dealing with complex T/C-only models. The ability to perform step-wise analysis better approximates nonlinear load-displacement curves and sets Pynite up for future nonlinear analysis features.

• Added P-Δ effects to vectorized results. Prior to this fix, member end forces and internal forces were being calculated correctly, but the P-Δ flag was not triggered in the methods that used vectorized results. Moment plots and moment arrays were the only items that were affected. Spot checking moments, and checking max/min moments were not affected.