[FIX] util: Fix bincount for object arrays

[lanzagar]

Issue

bincount failed for object arrays that contained np.nan, but worked on both float arrays with nans and object arrays without them.
This was a problem since metas are often stored as object arrays and contain primitive variables, for which bincount should work (even if there are missing values).

Description of changes

The array must contain numbers anyway (since it is later converted to int32 for np.bincount), so it should not hurt to cast it to float before checking for nans. Almost all arrays in Orange are either float or object anyway, so this should be a noop for float arrays and a fix for object arrays.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3831 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3831   +/-   ##
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  Coverage   85.06%   85.06%           
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  Files         374      374           
  Lines       69148    69148           
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  Hits        58821    58821           
  Misses      10327    10327