Loading and fitting a line in an SDSS spectrum with specutils

[eblur]

I produced two notebooks from the Astropy Spectroscopy Sprint (April 6 - 10) that are ready for review. When we are happy with them, I'll submit a pull request.

Part 1: Load, Plot, and Smooth an Optical Spectrum from SDSS

Part 2: Measure Properties of a Spectral Emission Line

[kelle]

From #260 , we want this tutorial to check all these boxes:

Measure EW of high res vs low res version.

• how to access spectrum? astroquery? high res spectrum with uncertainty.
• custom loader
• generate the Spectrum1D object.
• something about noise - can generate our own unc attribute of Spectrum1D.
• spectrum arithmetic.
• spectrum regions
• Deconvolve - smoothing.
• fit continuum, normalization.
• how to measure an EW already on Getting Started
• line flux.

I made a draft tutorial here which might be helpful:
https://colab.research.google.com/drive/1QQqFwv6QKK7wc1wiLq0KnjLncMeDBUUs