add guess_time() function to approximate the PMT hit time

This function is only used when the expected number of photons reaching a PMT
is *very* small. In this case, we still need to estimate the PMT hit time PDF
for indirect light which is modelled as a flat distribution starting at the
time where the PMT is most likely to be hit from direct light. Since we compute
the most likely time for a PMT to be hit from direct light by computing the
integral of the expected charge times the time and then dividing by the total
charge, when the total charge is very small this can introduce large errors.

Note that this code already existed but it was computed in the likelihood
function. This commit just moves it to its own function to make things look
nicer.

0 files changed, 0 insertions, 0 deletions