fixed GMeans trying to split clusters with one element

[rpsdm]

fixes #82

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (47bc534) 90.57% compared to head (94f7b38) 90.59%.

Files	Patch %	Lines
clustpy/partition/gmeans.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   90.57%   90.59%   +0.01%     
==========================================
  Files          59       59              
  Lines        7345     7348       +3     
==========================================
+ Hits         6653     6657       +4     
+ Misses        692      691       -1     

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