Major Update 6

Major Update 6 comes with improvements in API usability.

• native support of imputing numpy arrays is no longer supported. It made the code too complex.
• Mean match customization is now much more simple, and handled entirely through parameters instead of custom classes. The parameters mean_match_strategy and mean_match_candidates are all that are needed to control mean matching.
• The saving and loading of kernels was modernized to make use of getstate and setstate, without the need for a load_kernel method.
• Major improvements to testing suite.
• Plotting was moved to plotnine.