This PR moves logic for loading fluxes, cross sections, and detector models into the resources directory. With the idea that the logic for manipulating files, importing other packages, building tables, etc. should be relegated to python code that lives outside of siren itself. This changes the way we treat the tabulated fluxes a little bit since we don't expect the code within the resources directory to generate a file for us, but rather we expect it to return a siren.distributions.PrimaryEnergyDistribution object (that in this case happens to be a tabulated flux). This also involves essentially decoupling DarkNews from siren, since all the DarkNews-specific logic would then live within a file in the resources directory. Finally for the detector models, this means that a detector model can now take parameters since we're always calling a function to load the detector model rather than just pointing to a file.