Mugration weights

[trvrb]

Include an option to set equilibrium frequencies to mugration analysis in augur traits. This passes in a --weights option that specifies a tsv or csv file in the following format:

division	Hubei	50.0
division	Jiangxi	1.0
division	Chongqing	1.0
...

where the first column refers to the metadata field name, the second to the trait and the third to its value. This mirrors the format we use for colors.tsv and lat_longs.tsv.

This can be tested with this branch of nCoV: https://github.com/nextstrain/ncov/tree/phylogeo

This needs to wait on merge of #431.

[rneher]

The thing you changed in the last commit wasn't a typo. I had the weights file formated differently:

https://github.com/nextstrain/augur/blob/mugration_weights/tests/builds/zika/config/trait_weights.csv

we generally have to sort out how we deal with csv/tsv parsing. I think having a name in the first column is more natural. but I realize that we have it flipped in the colors file.

[trvrb]

Ah! This makes more sense. I think I'd prefer to have this mirroring of colors.tsv and lat_longs.tsv with weights.tsv. Are you okay with this structure?