Add a test on saving results

[bowni]

Follow-up of PR #337 , adding a unit test to it.

[codecov-commenter]

Codecov Report

Merging #358 (495814e) into master (eb20b80) will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
+ Coverage   79.21%   79.48%   +0.26%     
==========================================
  Files          15       15              
  Lines        3152     3158       +6     
==========================================
+ Hits         2497     2510      +13     
+ Misses        655      648       -7     

Impacted Files	Coverage Δ
mplc/dataset.py	75.67% <0.00%> (+0.11%)	⬆️
mplc/contributivity.py	76.55% <0.00%> (+1.03%)	⬆️

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