Add an example dataset = solar spectral irradiance

[zkbt]

It'd be nice to add a few example datasets that can be automatically downloaded, to make it easier for a new user to dive into some real data.

One good example would be solar spectral irradiance as a function of time.

• Pick a dataset (or a few) from https://lasp.colorado.edu/lisird/. It'd be nice to have one that spans a pretty wide range of wavelengths.
• Write a tiny little wrapper that downloads and reads it. It should use the astropy download tools, so the data file gets nicely cached. (That will speed up tests, and just generally make things more pleasant.)
• Write a documentation example that makes some cool plots with that dataset.

[zkbt]

@gmduvvuri, if you're interested in this (?), I think https://lasp.colorado.edu/lisird/data/lasp_gsfc_composite_ssi/ might be the most interesting dataset to go with. You could try to do something clever with the API, or you could just hard code one of the links to the complete datasets given at the bottom of the page. (I'm probably adding NetCDF as a dependency soon for another data format we want to read, but you can also always do the IDL sav file with scipy.io.readsav.)

Seeing a dyanmic spectrum of this dataset would be snazzy, and all that's needed to include it in the documentation is to write it up as a notebook!