Ensuring the unit circle of the Nyquist plot remains round, even after resizing the plot
using functools.partial to define sigmas as a sub-function of SVD
Include a function to return the input and output directions associated with the maximum and minimum singular values - i.e. direction of poles and zeros.

Standardise 'G' as the input matrix to all functions concerning SVD
sv_dir returns the maximum and minimum singular vectors, which are related to the directions of the poles and zeros
Merge pull request #100 from kendasareking/master

sv_dir(G): a function that returns the maximum and minimum singular vectors of G