Base flux model with free normalization param should accept input text file

[chiarabellenghi]

Add a new EnergyFluxProfile object in https://github.com/icecube/skyllh/blob/d76a49c3c6176c6fb6fc13e14f51ce81d7f9cd73/skyllh/core/flux_model.py.

The new class should accept text files with energy and flux columns.