This commit updates the python code to work with python 3 and with a
newer version of matplotlib.

- zip_longest -> izip_longest
- fix tick marks for log plots
- scipy.misc -> scipy.special

This commit adds two improvements to the quad fitter:

1. I updated quad to weight the random PMT hit selection by the probability
that the PMT hit is a multiphoton hit. The idea here is that we really only
want to sample direct light and for high energy events the reflected and
scattered light is usually single photon.

2. I added an option to quad to only use points in the quad cloud which are
below a given quantile of t0. The idea here is that for particles like muons
which travel more than a few centimeters in the detector the quad cloud usually
looks like the whole track. Since we want the QUAD fitter to find the position
of the *start* of the track we select only those quad cloud points with an
early time so the position is closer to the position of the start of the track.

Also, I fixed a major bug in get_hough_transform() in which I was using the
wrong index variable when checking if a PMT was not flagged, a normal PMT, and
was hit. This was causing the algorithm to completely miss finding more than
one ring while I was testing it.

This commit updates get_event() to flag PMT charges below qlo, which is the
minimum charge value that we compute the charge PDFs for. This is to prevent
the likelihood from returning nan.

This commit adds a new program called zdab-cat which is kind of like fit, but
just produces the YAML output without actually fitting anything.

This commit updates the test-find-peaks script to plot Cerenkov rings for each
of the peaks. It also updates the script to use quad to find the position
instead of using the MC information. Finally, I added a -n argument to the
script to specify how many peaks to draw.