Major update 6

[AnotherSamWilson]

Update to Major Version 6.0.0

Changes

• 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 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.