Create spectra.py to store spectra functions

[dr-rodriguez]

Create a spectra.py file to store spectra-related functions and classes. This would contain the code to take a URL and get a specutils spectrum object, among other things.

[dr-rodriguez]

A boiler plate implementation now exists that successfully loads an example Spex Prism file.

[dr-rodriguez]

Long term, we'll want to move these as pull request against specutils but for now this is done.