Fixes for minor issues discovered during work on demografr and other things

[bodkan]

A few (minor) issues have been discovered during the work on the ABC R package demografr as well as through in-person reports from users. Turns out throwing thousands of randomized models and models written by other people not involved with slendr at the package means it gets tested much more extensively! 🔥 :)

[codecov-commenter]

Codecov Report

Merging #130 (b72cad9) into main (b2c44a1) will increase coverage by 0.02%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   83.71%   83.74%   +0.02%     
==========================================
  Files           6        6              
  Lines        3065     3070       +5     
==========================================
+ Hits         2566     2571       +5     
  Misses        499      499              

Impacted Files	Coverage Δ
R/interface.R	80.10% <100.00%> (+0.20%)	:arrow_up:
R/tree-sequences.R	87.76% <100.00%> (ø)
R/visualization.R	61.74% <100.00%> (+0.14%)	:arrow_up:

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[bodkan]

@IsabelMarleen as discussed on Slack, I'm merging this now. Please re-run devtools::install_github("bodkan/slendr") to get the development version from the main branch to keep things in sync with your thesis code.

Also, it would be great if you could test what happens with your Gutenkunst slendr model generated by your import_demes() function as reported here. I suspect changes implemented in this PR won't make a difference there but still... worth a try? Regardless of the outcome, could you please post an update note under that issue? Thank you. (Not urgent!)