Simultaneous sources in pulse events

[apragsdale]

WIP. First pass to update existing code and tests to handle sources and proportions as lists instead of str/value. See popsim-consortium/demes-spec#100 for discussion.

Need to expand testing of valid and invalid pulses with multiple sources.

[codecov]

Codecov Report

Merging #353 (a14a593) into main (586f073) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head a14a593 differs from pull request most recent head d59c466. Consider uploading reports for the commit d59c466 to get more accurate results

@@           Coverage Diff           @@
##             main     #353   +/-   ##
=======================================
  Coverage   99.93%   99.93%           
=======================================
  Files           5        5           
  Lines        1502     1518   +16     
=======================================
+ Hits         1501     1517   +16     
  Misses          1        1           

Impacted Files	Coverage Δ
demes/demes.py	100.00% <100.00%> (ø)
demes/ms.py	100.00% <100.00%> (ø)

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[grahamgower]

LGTM @apragsdale. Just a couple of minor code nits.

[jeromekelleher]

Changes basically LGTM, but I haven't gone through the fine details.

[apragsdale]

Thanks for the suggestions, @grahamgower. I think everything is looking reasonable here after rebasing from the merge of #362. Will squash if you agree.

[grahamgower]

Looks good @apragsdale. Sorry for giving you more work, with all the merge conflicts!

[apragsdale]

No problem at all! Thanks @grahamgower. I'll squash and merge after addressing your comment.