Fix the solution sampling code

[nathanaelbosch]

Currently its output is incorrect. This PR should fix it. Note that the solution sampling is undocumented so this PR should not affect any public API.

Apparently dense sampling is broken! This is most likely due to how diffusions are handled by the interplation->sampling part. Probably, the time-varying diffusions should be stored in such a way that it's clear which ones should be active for which interval, directly in .diffusions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.88%. Comparing base (2f2f6ad) to head (39d389c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
+ Coverage   91.84%   91.88%   +0.04%     
==========================================
  Files          44       44              
  Lines        2219     2219              
==========================================
+ Hits         2038     2039       +1     
+ Misses        181      180       -1     

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