Fix summaries to add percent colored by category

[ElDeveloper]

The Legend class should compute percentages colored by each category and show as part of the summary.

[fedarko]

Given the current status of the codebase, it might be preferable now to have Empress pass the output of _projectObservations() to the BPTree object, which would I guess compute percentages from that using the node keys contained within it. This percentage data could then be passed to the Legend class, which would just need to have its categorical legend functionality expanded a bit to display this data (maybe with an optional percentageCovered parameter or something).