Add options to select the type of interpolation in the arguments

So far the interpolation used by default is a cubic interpolation of the flux values of (log10(E),costheta) https://git.km3net.de/simulation/NuFlux.jl/-/blob/main/src/NuFlux.jl?ref_type=heads#L254 We would like to allow some flexibility on the choice of interpolation.