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.
I have more of a problem with the fact that 551 and 556 are effectively the same. The only difference is the test to see if the func is None. Also, 551 will also fail if len(func) is greater than len(eq), so the error isn't exactly correct.