At present, [stft](https://github.com/IoSR-Surrey/untwist/blob/master/untwist/transforms/stft.py#L111) does not allow for zero-padding, i.e. the size of the analysis window is fixed to `fft_size`. Goal: Allow for (preferably [zero-phase](https://ccrma.stanford.edu/~jos/sasp/Zero_Phase_Zero_Padding.html)) zero-padding of the windowed data.