Turn boxed least squares tutorial into a full tutorial here

[bsipocz]

@dfm has made this tutorial for the new bls module in astropy. Ideally with a little bit of extension it should end up with the other tutorials here.

https://gist.github.com/dfm/96e30d891c0d351daae9aaaef56d3e78

Extensions I had in mind:

• use astroquery to get the kepler data from MAST
• use Table and its columns rather than getting data directly from the fits HDU.
• maybe also run LS on the same light curve to compare to the BLS results, but this is not strictly necessary.

[kelle]

I've made a colab version of this notebook here so we can edit:
https://colab.research.google.com/drive/1nqY0FRCkQLOkYzYfKV1R3dAuos_dG-i_

[kelle]

Updated link to notebook: https://colab.research.google.com/drive/1_9t1PM-oyOyiDruLxEx0J7djSsX163qC?usp=sharing