Add a figure to one of the vignettes demonstating subtaxa, supertaxa, leaves, etc

[zachary-foster]

This was suggested by one of the reviewers of the F1000 paper on taxa.

This is something I have considered in the past, but have not made a good version of yet. I am thinking of a set of heat trees, all with the same shape, with sets of branches highlighted showing which taxa are included in the subtaxa, supertaxa, leaves, roots, stems, internodes, etc. Could also highlight what various filter_taxa commands would remove if applied.

[sckott]

seems like a good idea