Newexample/vanderpol

Implements the van der Pol oscillator as new example of a nonlinear ODE. Uses a tailored Newton method in impl_solve.

Because the vdP oscillator is originally a second order system (and for parameter nu=0 becomes the linear oscillator), this might also be of interest for the second order SDC.

The branch contains a test as well, but so far only for nu=0 (i.e. the linear case), because otherwise there is no analytical solution and I did not come up with a nice test here...