Sysode docs

It contains the docs of all implemented method to solve for system of ordinary differential equations which include 
- linear system of 2 equations of 1st order,
- linear system of 2 equations of 2nd order,
- linear system of 3 equations of 1st order,
- linear system of n equations of 1st order,
- non-linear system of 2 equations of 1st order