split up the track integral into two intervals

This commit updates the likelihood calculation to split up the track integral
into two intervals in some cases. I noticed when fitting some events that the
likelihood value would change drastically for a very small change in the fit
parameters. I eventually tracked it down to the fact that the track integral
was occasionally returning a very small charge for a PMT which should have a
very high charge. This was happening because the region of the track which was
hitting the PMT was very small and the cquad integration routine was completely
skipping it.

The solution to this problem is a bit of a hack, but it seems to work. I first
calculate where along the track (for a straight track) the PMT would be at the
Cerenkov angle from the track. If this point is somewhere along the track then
we split up the integral into two intervals: one going from the start of the
track to this point and the other from the point to the end of the track. Since
the cquad routine always samples points near the end of the intervals this
should prevent it from completely skipping over the point in the track where
the integrand is non-zero.

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