Initial proposal for a STFT function

Since adding both the PSD and STFT coefficients to the `Spectrogram` type would make it need a lot more memory than it needs now, I thought it would be better to have a separate function for the STFT. As you will notice, it is almost exactly the same as `spectrogram`, except that it does not compute the PSD and returns just a matrix instead of an immutable type (the latter could of course be changed to a STFT type). Do you think we should do it like in this PR or anything different (e.g., reusing the same spectrogram type/function)?