Cleaning up finished tutorials and updating README files

[miguelglezb]

Renaming some of the old tutorials and removing empty notebooks from the repo meant to be placeholders. Given that the source code has been published, it is important to tidy up the tutorials and keep only the relevant information.
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[PabloCorcho]

A good place to show cube.get_white_image()

[PabloCorcho]

In rss_registration.ipynb (I cannot comment on the file):

• First show the keywords of info and the the content?
• Science fibres are not fibres without bad pixels, but fibres that are located on the target object (some IFUs have additional fibres that are placed in blank spaces to measure the sky emission simultaneously).
• I'm puzzled to see that "read" is a correction XD... We could implement an origin attribute which either returns the path to the file from which it was read, or None, implying it doesn't come from any file (e.g. mock data).
• You say "Make sure to use astropy quantities and not simple float values.", but then you prove that the code works with floats. I would explicitly mention that, when using floats, the method assumes the range is in AA.
• When you use the method from_fits, I would make very clear that the method only works with FITS files generated by PyKOALA, whereas for reading native data from multiple instruments we use the instruments module.
• If you use methods with default values, perhaps is good to also include a cell with the help information so the user knows what parameters can be tweaked.