Create stratified collectors curves

[lwaldron]

I would be very interested to know how complete the bugsigdb.org coverage is of taxa at the species and genus, per major body site. Body sites should be grouped into major ontology terms, for example oral + nasopharynx + orpharynx, digestive tract, skin, urogenital tract (will have to look up what those terms are, but would be done using the bugsigdbr::subsetByOntology function. Plotting can be done using the BugSigDBStats:: plotUniqueMicrobesOverTime function. This could be a new vignette since there would be many plots. This follows #10