Create a method on Spectrogram for cropping, both by time and frequency

[hayesla]

Describe the feature

It would be useful to be able to crop a Spectrogram over a region of interest, such as time or in frequency. An example would be the fact that a lot of sources provide daily spectrogram data and you're only interested in a time region close to a flare etc.

You may also want to crop by frequency.

Proposed solution

Scope for this? if so should it just be .crop_freq() or .crop_time() or truncate()

[samaloney]

Do you want to actually crop the entire object or only for plotting? If it's to crop the entire object this would push me towards using ndcube as a base then that could accomplish that with wcs and crop ...