0.6.1 release

New features in this release:

• Storing computed eigenvalues to avoid re-calculating them later. They are just computed again if the matrix is re-sampled.
• The method rvs of the class WignerSemicircleDistribution now uses the relationship between the Beta distribution and the Wigner's Semicircle law to generate random samples of it.
• The method plot_empirical_pdf of the class WignerSemicircleDistribution now uses the class method rvs to generate the sampled eigenvalues, instead of sampling eigenvalues from an instance of the GaussianEnsemble.
• Updated doc-strings of the updated classes and methods.