Minor things

[petrelharp]

The first thing here is pending resolution of #676 (i.e., "can we reload into SLiM after converting alleles"; it currently passes on my machine with MesserLab/SLiM#680 but won't pass CI until that's merged into multitrait (if it is).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.69%. Comparing base (49f8dd0) to head (42cb0ab).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #454   +/-   ##
=======================================
  Coverage   96.69%   96.69%           
=======================================
  Files           8        8           
  Lines         908      908           
  Branches      181      181           
=======================================
  Hits          878      878           
  Misses         18       18           
  Partials       12       12           

Flag	Coverage Δ
python-tests	96.69% <100.00%> (ø)

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