Update the notebook to reconstruct a source flux

[YoshikiOhtani]

With this pull request I updated the notebook check_sed_from_dl3.ipynb to reconstruct a source flux as follows:

• Add the code to reconstruct the light curve at the end of the notebook.
• Remove the support for the older version of gammapy, since it is best to use the new feature, especially WobbleRegionsFinder and using dynamic theta cuts.
• Update the style to improve the readability and make it user-friendly.
• Rename from "check_sed" to "check_flux" since I added the code to reconstruct the light curve.

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