add function to sample scintillation

[ManuelHu]

1. Include the fano factor in the ScintConfig object, to bundle all properties relating to the amount of photons released
2. Povide a function utils.scintillate(...) to return an array of timestamps at which scintillation photon emission happens. This is strictly following Geant4's scintillation model

Example usage:

import pint

import legendoptics.lar as lar
import legendoptics.utils as utils


u = pint.get_application_registry()

times = utils.scintillate(
    lar.lar_scintillation_params(),
    lar.lar_lifetimes().as_tuple(),
    "electron",
    1 * u.MeV
)

plotting a histogram of times gives the usual two-component exponential (x-axis is time in nanoseconds):

Open discussion points

• Add a way to interact with scikit-hep/particle for conversion PDG id -> geant scintillation particle name (this is not a simple lookup table...)
• Geant also includes sampling a time and spatial offset calculation along the path segment (i.e. it assumes the energy deposition is uniform along the path, and not localized at its end). This is not yet implemented here, and would require a much more sophisticated output scheme (including start & end timestamps + coordinates for each segment with energy deposition)