SIMP topology optimization complex sensitivity

[manavbhatia]

I have merged #14 into master to demonstrate topology optimization. The code runs fine in serial using Eigen data structures. This example demonstrates the capability, but currently needs some more work:

• Add documentation with convergence plots and optimal topology
• Verify complex-step sensitivity: While the analytical sensitivity is fine and is able to drive the optimizer, the complex-step sensitivity is giving inaccurate results. This needs to be checked.

[manavbhatia]

Second issue fixed with #54