Review of utils/neutrino_astronomy.py

[mlincett]

Pull request following up on #236.

Drops all the "nearest source" astronomy calculations, leaving that exercise to the user.

The calculate_astronomy() function now only calculates the integrated energy flux from the differential flux.

Although there seems to be a problem, since the functionality was used in some past analysis code, following up again in #236.

[mlincett]

The operations and outputs based on "Mean Luminosity (erg/s)" (as in "mean luminosity of the nearest source") have been removed from calculate_upper_limits() in the core.unblinding module. This function is not covered by unit tests nor it is invoked in any of the code in the repo (including analyses). I will consider moving such function to utils in a future PR, as it is not an organic part of the unblinding code.

Unless there are further comments / doubts I will proceed with the merge tomorrow.