Non-linear system of two ODEs of 1st order

This PR contain the methods for nonlinear system of ODEs for 2 equations. This also includes all test cases for these methods and also test cases for classify_sysode.
When I look at the pull request, it says that it's free to be merged. So, I'm assuming it's passing tests. Your docs pull request isn't passing, so I'll take a look at that.