add country-level citation graph per topic

[Daniel-Mietchen]

First attempt at #1101.

Worked for all the about two dozen topics I tested, except for https://tools.wmflabs.org/scholia/topic/Q3450985 (neurodevelopmental disorders), which gave an empty response.

[Daniel-Mietchen]

Another quirk: for microstructure, the flag for South Africa was just shown as the flag from the Apartheid period. This was easily fixed but should be kept in mind.

Also, while tracking this down, I was reminded that we have flag image (P41), which will likely make the query faster.

[Daniel-Mietchen]

The solution to the question as to why https://tools.wmflabs.org/scholia/topic/Q3450985 (neurodevelopmental disorders) did not yield any result was simply that there were too few topic annotations (15 right now).

I am starting a first batch towards changing that.

[egonw]

I checked the tox report and there are no problems. I still have trouble interpreting the arrows, and I am a bit worried that it will overemphasize the work in the USA and I'm afraid it will further stress the "winner takes all" situation in science (no direct reason to not accept it).

[fnielsen]

I am unfamiliar with the target: notation.

[Daniel-Mietchen]

@fnielsen See #431 for the target: notation.