update shower position distribution parameters for muons

This commit updates the a and b parameters for the gamma distribution used to
describe the position distribution of shower photons produced along the
direction of the muon. Previously I had been assuming b was equal to the
radiation length and using a formula from the PDG to calculate a from that.
However, this formula doesn't seem to be valid for muons (the formula comes
from a section describing the shower profile of electrons and gammas, so it's
not surprising). Therefore, now we don't assume any relationship between a and
b.

Now, the value of a is approximated by a constant since I couldn't find any
functional relationship as a function of energy to describe a very well (and
it's approximately constant), and b is approximated by a single degree
polynomial fit to the values I got from simulating muons in RAT-PAC as a
function of energy.

Note that looking at the simulation data it seems like the position
distribution of shower photons from muons isn't even very well described by a
gamma distribution, so in the future it might be a good idea to come up with a
better parameterization.

Even if I stick with the gamma distribution, it would be good to revisit this
in the future and fit for a and b over a wider range of energies.

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