Plan for species page-rank service

[arlin]

For the portal and other applications, we would like to down-sample taxa by "importance" as defined by something like a page rank. @hyanwong has collected this kind of information.

The goal of this ticket is to come up with a plan to turn this kind of data into a web service backed by a database that is updated periodically. There are millions of species but possibly we do not need to know about the bottom 50 % of popularity.

In response to the query "mammalia+200", the service would return the top 200 mammal species.

[arlin]

we discussed this with @hyanwong on the 14th of April as per the notes here: https://docs.google.com/document/d/1mgal-qCKOlGGrHC0EsEve2KzxP_LCy_q2Y2_ytOeM6c/edit#heading=h.k36zpzey9z1n

Yan has code for this. We did not make a decision about whether to implement or not. However, the idea still seems promising.