Support aggregating continuous-value barplots based on clade collapsing

[fedarko]

This came up in this morning's meeting (thank you @bextra!). The idea would be something like -- if a barplot layer using just continuous fields is selected, and clade collapsing is applied, there should be an option to combine all of the tip bars in that layer within each collapsed clade into a larger bar spanning the same width.

The color (or length) of these "aggregate" bars should be computed based on some combination of the tip values: average, sum, median, min, max, etc. The logic to do these computations should be reusable between this and the quantitative sample metadata aggregation stuff planned in #353.

So in the moving pictures example below (using phylum-level collapsing and showing a barplot of each tip's Confidence), it should be possible to visualize the average (or sum, median, etc.) confidence for each of the collapsed clades: which may be more useful than the current representation, which only shows confidence at the level of each tip.