Non-linear system of ODEs

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.
Removed a repeated condition

Condition for number of equation being equal to number of function was repeated. This commit contain removal of one of them.
Merge pull request #7676 from KDMOE/sysode_nonlinear_order1_2eq

Non-linear system of two ODEs of 1st order