daemonflux discussion

I've started with a possible implementation of reading flux tables produced with daemonflux in make_fluxes following the 2D interpolation ideas from the flux libraries (in that case, ROOT TGraph are used).

the project is currently a personal one, here https://git.km3net.de/lfusco/DaemonFlux.jl but I've started testing it in toy analysis scenarios

the idea is to load the tables that are provided in csv format from make_fluxes as dataframes, use a 2D interpolator in (logE, angle) and then return the flux at any energy/angle (note: the tables is binned in logE - cos(angle) for downgoing direction only, up/down symmetry is assumed)

it still requires a bit of fixes (e.g. angles are in degrees, not in radians, there is a "zenith-averaged" option for the output that I'm using for checks but should not be used for event weights, particles are spelled out and not listed by PDG ID) but tests look OK so far (see validation and tests for the moment)

final note: this is my first serious attempt to produce some useful code in julia, so any suggestion and comment is really welcome. I don't know if this is any ready to be implement in NuFlux.jl but this issue can be useful for discussion