Flexibly disambiguate multiple publications by the same author

[huddlej]

Adds a function to convert a zero-indexed article count for a given author to a
alphabetical disambiguation suffix. For example, two articles by Bedford et
al. get disambiguated as "Bedford et al. A" and "Bedford et al. B".

The new function is slightly overkill in that it supports an infinite number of
articles per author. We could easily unroll the first two loops into redundant
code that only works for a fixed number of articles (702), but the idea here is
that we never have to touch this function again.

One disadvantage of the alphabetical code is that articles with "AA" get sorted
next to "A" and before "B". A real world example is shown below where the author
had 85 (!) different publications.

Fixes #571

[codecov]

Codecov Report

Merging #581 into master will increase coverage by 0.16%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
+ Coverage   25.10%   25.27%   +0.16%     
==========================================
  Files          33       33              
  Lines        5178     5191      +13     
  Branches     1299     1300       +1     
==========================================
+ Hits         1300     1312      +12     
- Misses       3830     3831       +1     
  Partials       48       48              

Impacted Files	Coverage Δ
augur/export_v2.py	10.00% <86.66%> (+1.95%)	⬆️

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[rneher]

thanks, @huddlej. This looks good to me. I don't think we'll run into this A, AA, AB, B, C issue all too often. Having this generic solution is good IMHO.

[huddlej]

Great, thank you, @rneher! Merging now.