update guess_energy()

This commit updates guess_energy() which is used to seed the energy for the
likelihood fit. Previously we estimated the energy by summing up the charge in
a 42 degree cone around the proposed direction and then dividing that by 6
(since electrons in SNO and SNO+ produce approximately 6 hits/MeV). Now,
guess_energy() estimates the energy by calculating the expected number of
photons produced from Cerenkov light, EM showers, and delta rays for a given
particle at a given energy. The most likely energy is found by bisecting the
difference between the expected number of photons and the observed charge to
find when they are equal.

This improves things dramatically for cosmic muons which have energies of ~200
GeV. Previously the initial guess was always very low (~1 GeV) and the fit
could take > 1 hour to increase the energy.

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