Make matplotlib a dependency?

[kelle]

For the spectrum utils, one of the functions uses matplotlib. I made it so that matplotlib is not a requirement. If the user doesn't have it installed, it will let them know plotting won't work. But that brings up a larger question: do we want to make matplotlib a requirement? I could imagine other plotting/visualization scripts that could eventually live here.

[arjunsavel]

matplotlib is a big dependency, but I think that plotting is definitely within scope for this package. I'd be down to make it a dependency.