Big FITS file tutorial

[ceb8]

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This is an advanced tutorial that shows how to create a FITS file from scratch that is too large to fit in memory.

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[ceb8]

I'm not sure how to deal with the timing out issue. The point of this tutorial is to create an extra-large FITS file, so several of the operations to indeed take a bit of time, and can't really be modified not to.

[ceb8]

@embray Would you be willing to review this PR? I believe you were the author of the build-big-fits-file gallery example I built it off of and reference in the description (which I realized has now moved location, so I will update that).

[embray]

I'll try to take a look

[ceb8]

@embray thanks so much! No rush.