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.
On line 563 of ode.py you raise a Not Implemented error if the type of equation is None. So, I'm guessing that there's something about the equation that is causing problems for your classifier code. I'd recommend examining that output.