Short-Time Fourier Transform (STFT) implementation

[martonvavrik]

A (continuous) Short-Time Fourier Transform STFT shall be implemented, in a similar manner to APSD and CPSD. First it should be implemented in a separate time_frequency_analysis.py file, then the functionality should be added into the DataObject too.

[martonvavrik]

To follow the apsd's implementation, I also changed the standalone stft function to _stft (which made it internal). Should I keep this?

[sandorzoletnik]

Yes, this is OK.

[gergopokol]

It looks like this is already merged to master. Can we close this?