specutils tutorial on using sdfits files for spectra

[teuben]

During the specutils sprint week my notebook demo on https://github.com/teuben/gbtoy/blob/master/nb/ngc5291-demo3.ipynb needs a bit of work and then it can mature to a tutorial:

• Collection, not List
• example with dask to show speedup?
• clean up reduction in specutils portion
• alternative math to solve for calibration (potentially not in this notebook, but should be noted)

[eblur]

I have made a copy of this notebook in Google drive so it can now be edited in Colab

https://drive.google.com/file/d/1OeNoh0Uqk1NA1Jbr3UALqtEiHDaAStvb/view?usp=sharing

I started working on putting it into the Learn Astropy format, and I will stop short of loading the Spectrum1D object for now.