maturity-index: make heatmap optional

[nbokulich]

Improvement Description
Heatmaps are sooooo slooooow when there are many features (as is common). Waaay slower than the actual heavy lifting (supervised regression) in the maturity-index pipeline. Many users may not want to bother plotting these if they have other plotting methods in mind. This would also really speed up unit tests... Make optional?

Current Behavior
My hair turns grayer each time I need to wait for heatmaps to be produced by this plugin.

Proposed Behavior
Make heatmaps optional.

Questions

1. How can these be made optional?
   • generate an empty index.html? (ugly)
   • produce a quick-and-dirty heatmap in seaborn (this is how the old viz did things and it was so much faster, but impenetrable to read when you have hundreds to thousands of features)
   • Wait for optional outputs.