Fix broken notebooks in integration tests.

[sybenzvi]

During the tidying up of PR #417, we found several integration tests failing due to problems with the following four notebooks:

• AnalyticFluence.ipynb: several pandas table conversion problems with the SNOwGLoBES module.
• SNOwGLoBES_models.ipynb: most calculations are broken.
• Detector_demo.ipynb: data output shape errors.
• ExtendedCoolingTail.ipynb: missing abstract method _get_initial_spectra_dict.

[sybenzvi]

AFAIK, fixing these integration tests is one of the final hurdles to releasing SNEWPY 2.0. Can we get some eyes on this?

[jpkneller]

This is indeed the last thing to fix. I saved it until after the we decide what to do with the snowglobes module