[MaxVar split, Part 2] Added the visualisation improvements.

[perdaug]

Summary:

• Improved the interactive plotting;
• Improved the quality of plot_state(...);
• Migrated the pair-wise plotting to plot_state(...).

Please make sure

• You have updated the CHANGELOG.rst
• You have updated the documentation (if applicable)

Copyright and Licensing

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By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD3 (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)