ENH: skip k=1 in K-Means implementations

[martinfleis]

Closes #17

To-do:

• test cuML

[codecov-commenter]

Codecov Report

Merging #18 (2881920) into main (5cb041b) will decrease coverage by 0.71%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
- Coverage   62.85%   62.13%   -0.72%     
==========================================
  Files           3        3              
  Lines         603      618      +15     
==========================================
+ Hits          379      384       +5     
- Misses        224      234      +10     

Impacted Files	Coverage Δ
clustergram/clustergram.py	78.67% <33.33%> (-2.14%)	⬇️

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