Add istft

[takenori-y]

No description provided.

[codecov]

Codecov Report

Merging #22 (8c9b893) into master (51c0433) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files         146      148    +2     
  Lines        3385     3425   +40     
=======================================
+ Hits         3377     3417   +40     
  Misses          8        8           

Flag	Coverage Δ
unittests	99.76% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
diffsptk/core/c2acr.py	100.00% <ø> (ø)
diffsptk/core/gnorm.py	100.00% <ø> (ø)
diffsptk/core/ignorm.py	100.00% <ø> (ø)
diffsptk/core/levdur.py	100.00% <ø> (ø)
diffsptk/core/__init__.py	100.00% <100.00%> (ø)
diffsptk/core/delay.py	100.00% <100.00%> (ø)
diffsptk/core/istft.py	100.00% <100.00%> (ø)
diffsptk/core/stft.py	100.00% <100.00%> (ø)
diffsptk/core/unframe.py	100.00% <100.00%> (ø)
diffsptk/misc/utils.py	100.00% <100.00%> (ø)
... and 2 more

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