add a user-friendly assert that catches the NaNs

[exaexa]

...before they cause a really weird error elsewhere.

This is simple and hopefully mergeable part taken out of #67 and #68 (this change is correspondingly removed from there)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.57%. Comparing base (dd9768e) to head (d65e802).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   72.56%   72.57%   +0.01%     
==========================================
  Files          71       71              
  Lines        2497     2498       +1     
==========================================
+ Hits         1812     1813       +1     
  Misses        685      685              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[davibarreira]

Thanks, @exaexa !