Fixing various gene flow errors

[bodkan]

This finally addresses inconsistent error messages formally reported in #132.

Also simplifies and cleans up the gene_flow() code a bit in the process. No breaking changes or changes to the user-facing interface introduced by this (but I hope some yet-undiscovered issues will be correctly caught as a result of this).

[bodkan]

@IsabelMarleen: Still have to do more testing but I'm afraid it's already clear we will have to do the "gene flow happens earliest at +1 (or -1, for backwards time models) time point post split" as we discussed a couple of times.

For your demes project, issue a warning wherever an adjustment of this kind needs to be made during compilation from demes into slendr.

(This is not a priority though! Unless that's something that absolutely has to happen urgently, let's deal with this fully after your thesis is submitted. Focus on your 🐘 now. :))

[codecov-commenter]

Codecov Report

Merging #134 (d7fad95) into main (1473227) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
- Coverage   83.77%   83.75%   -0.02%     
==========================================
  Files           6        6              
  Lines        3075     3072       -3     
==========================================
- Hits         2576     2573       -3     
  Misses        499      499              

Impacted Files	Coverage Δ
R/interface.R	80.16% <100.00%> (-0.10%)	⬇️
R/utils.R	88.08% <100.00%> (ø)

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