Integration of astropy into spectra

There are many flaws in this change. Please give me your reviews, comments to make this better! 
I have made the changes for callisto but not for swaves. As there is no test for `swaves.py` I tried to manually open a swaves file using `SWavesSpectrogram` but it shows error `Index error: list index out of range` (even in master branch). Is that faulty or am I doing anything wrong?