Sysode linear 1storder 3eq

This PR implements linear system of three equations of first order with coefficients being both constant and function of variable. This PR is also about implementing 'n' homogeneous equations of first order with constant coefficients.
@flacjacket @tjl How to make this work? I defined startnum as global and I want to retain the value of startnum even it exits from this function ```eigen_vector``` and use it in next call of ```eigen_vector```. I tried using defining startnum as global (which should avoided) but still it doesn't work. Any idea?