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.
Yeah, I want to compare ```len(set(func))``` to ```len(eq)``` and if that is the case I want to change ```match['func']``` thats why I modified it like that.

Oh, I will remove L551.