Problem 798

It would be nice if there were a way to produce a uniform external source using
the GPS. This would be useful, for example, for simulating cosmic muon spectra
or external gamma rays.

The idea is as follows: the user would provide the radius R of a bounding sphere
for his/her volume. When it is time to generate an event, first a momentum
direction p^hat is simulated (according to whatever angular distribution the
user prefers). Then the position of the event is chosen randomly on a disk of
radius R whose normal points in the direction p^hat and whose center is at the
vertex -R*p^hat. This gives a uniform flux of events impingent on the detector
in the direction p^hat.

Jason