ENH: from_data and from_centers methods

[martinfleis]

Addind the ability to create clustergram using custom data, without the need to run any cluster algorithm within clustergram itself.

from_data gets labels and data and creates cluster centers as mean or median values.

from_centers utilises custom centers when mean/median is not the optimal solution (like in case of GMM for example).

Closes #10

[codecov-commenter]

Codecov Report

Merging #12 (bc354b5) into main (35bbf8d) will increase coverage by 2.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   57.59%   59.81%   +2.21%     
==========================================
  Files           3        3              
  Lines         507      535      +28     
==========================================
+ Hits          292      320      +28     
  Misses        215      215              

Impacted Files	Coverage Δ
clustergram/clustergram.py	79.20% <100.00%> (+2.62%)	⬆️

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