Modified some small things, such as: adding a few more selection
options, updating Mutation and Crossover to use the new options, and
adding a comment in Abstract Node.
Fixed a bug in SubtreeSelection, it now works properly.
Started writing a Symbolic Regression Problem for Sin(x). Changed a
few minor things while writing it so that everything fit together
nicely.
Finished writing SymbolicRegressionSinX and its tests.