[fixes #20] Implement Inverse Short Time Fourier Transform

[SiboVG]

This PR implements the inverse short time fourier transform.

Additionally, I also implemented:

• a method in _Window for applying the inverse of a window (sometimes, you want to undo the effect that windowing has done on a signal, for example I needed to do inverse windowing for the ISTFT)
• Changed some naming on return methods in DFT and IDFT, which seemed more standard and concise to me
• implement phase returning for IDFT