Closed solution polynomials only

I have removed the Bessel calculation from the library which has been not very precise and bloated the code. Only those filters are implemented which have a closed solution for the poles / zeros: Butterworth, Chebyshev and RBJ.

Better to get the zeros/poles from Python's scipy with reproducible results. No point of finding the roots of polynomials every time the filter starts up with the risk of not converging.

See the example in iirdemo how to design in Python and then copy/paste the coefficients over.