New testing function

Added is_twelfth_int() to test if an array is made of integers divided
by 2, 3 or 4 (e.g. if it's made of averages of low numbers of ints).
Later used to see the binning of a feature should be around integers.
Bin a feature around integers if it should be
Fix binning for variation of <#phonemes>

The FeatureAnalysis class now sets binning to 'around integers' if the
feature obviously comes from averages of a low number of integers (see
previous commit). Using this, the <#phonemes> is rebinned for the
feature variation plots.

Finally, substitutability plots are rescaled to [0, 1] (instead of
[-0.1, 1.1]) and the top and bottom lines were removed.