fix: make sure int values of boxlength are converted to floats

[DanielaBreitman]

When I input an int value of boxlength to get_power I get a very different (and wrong) power spectrum.
We should just include all the input types for all functions, but in the meantime, I added a quick fix where 0.0 is added to the boxlength to make sure it is a float.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.88%. Comparing base (089bf2e) to head (b807871).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
+ Coverage   92.86%   92.88%   +0.01%     
==========================================
  Files           5        5              
  Lines         575      576       +1     
==========================================
+ Hits          534      535       +1     
  Misses         41       41              

Flag	Coverage Δ
unittests	92.88% <100.00%> (+0.01%)	⬆️

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[steven-murray]

Thanks @DanielaBreitman. The preferred way to get float-type values is to use the float(val) syntax, which is also more explicit, so let's use that.