Euclid: Simplify spectra notebook

[bsipocz]

This PR:

• swaps PyVO to astroquery
• uses astropy API consistently
• cleans up pandas usage as there is nothing specific it is required for
• relies on astropy Quantity support for plotting
• removes usage of BytesIO as fits.open is fully capable of dealing with URLs
• Cleans up the narrative a little bit to make code content and text more consistent

Please check the rendered HTMLs for an easier view.

[bsipocz]

This now also includes changes from #89