Fix float block index causing hang due to infinite recursion

[KnutAM]

Calling BlockIndex((1.0, 2.0), (1,2)) currently causes an infinite growth of the tuple b, which this PR fixes by requiring tuple a to have integer elements. This lead to the computer just working for a very long time (at least 10 minutes) without any error message.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.53%. Comparing base (7ebbea7) to head (4c8ad37).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #492   +/-   ##
=======================================
  Coverage   94.52%   94.53%           
=======================================
  Files          19       19           
  Lines        1789     1810   +21     
=======================================
+ Hits         1691     1711   +20     
- Misses         98       99    +1     

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[mtfishman]

Good catch, thanks for fixing that.

[KnutAM]

Thanks for quick response and merge!