Add support of selecting different from subsets proportionately

[FanwangM]

The original implementation does not support selecting different subest proportionately. This PR addresses that issue.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.17%. Comparing base (90acdfe) to head (fafc517).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
selector/methods/base.py	83.78%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #270      +/-   ##
==========================================
- Coverage   95.37%   95.17%   -0.21%     
==========================================
  Files           9        9              
  Lines        1017     1037      +20     
==========================================
+ Hits          970      987      +17     
- Misses         47       50       +3     

Files with missing lines	Coverage Δ
selector/methods/distance.py	98.12% <100.00%> (+0.67%)	⬆️
selector/methods/partition.py	98.03% <ø> (ø)
selector/methods/base.py	88.67% <83.78%> (-5.77%)	⬇️

[FanwangM]

Some testing examples.