Add learnable option to MDCT

[takenori-y]

From #123

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.54%. Comparing base (380f782) to head (7988a5e).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #125   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files         244      244           
  Lines        8725     8758   +33     
=======================================
+ Hits         8685     8718   +33     
  Misses         40       40           

Flag	Coverage Δ
unittests	99.54% <100.00%> (+<0.01%)	⬆️

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