Added damping parameter eta

[MagnusSaurbier]

As presented in Section Damping in
Assyr Abdulle: Explicit Stabilized Runge-Kutta Methods https://www.epfl.ch/labs/mathicse/wp-content/uploads/2018/10/27.2011_AA.pdf
it is useful ensure, that there exists a small stable region around each eigenvalue, for which the method has been optimised.
This can done by demanding
|R(z)| \leq 1-eta for some small eta > 0
instead of just |R(z)| \leq 1.

I have added this technique to the poly_opt_bisect function. I am not too familiar with Matlab common practice though, so the parameter input scheme I chose might not be ideal. Feel free to adjust this.