Fixes #10 (Optimizer::getAlgorithm)

Each implementation of OptimizerSystem implements getAlgorithm. Something I'm not too sure about is my use of SimTK_APIARGCHECK_ALWAYS (https://github.com/chrisdembia/simbody/compare/issue-10?expand=1#diff-829061f3d46e7762cad70320fdaf5d1bL39).

I wrote a test that ensures getAlgorithm returns the right thing for the systems tested for IPOPT, LBFGS, and LBFSGB.