Fix unintentional defining of multiple datasets

[jameshadfield]

The original design of nextstrain narratives was that each "section" (paragraph) should define its own dataset, such that a single narrative could display different datasets (or the same one if each section specified the same dataset). This was never implemented -- i.e. in auspice currently only the dataset specified in the YAML frontmatter is ever used -- which has resulted in the inadvertent creation of narratives which specify different (and perhaps invalid) datasets.

Nearly every narrative file in this repo had inadvertently defined multiple datasets! Often this appears to be due to copy&paste of paragraphs from previous narratives etc. Auspice will soon "correctly" interpret these multiple datasets (nextstrain/auspice#1164).

This commit changes 206 narratives which previously specified multiple datasets so that each one now only specifies the dataset defined in the YAML frontmatter. This will preserve the current behavior in future versions of Auspice. The only exception is the "test_multiple-datasets.md" narrative.

There is also a small script which can be run periodically to identify narratives which define multiple narratives.

Closes #8

[cassiawag]

Thanks for pointing this out, James, and fixing all of them. I hadn't realized how rampantly we've defined multiple datasets. I'm tagging @sidneymbell here too, so that she's aware as well.