Track energy loss variations within a hit segment

[ClarkMcGrew]

The current implementation of ionization saturation (i.e. Birks saturation) assumes that the calculation is going to be done internal to edep-sim. It would be very good if the saturation calculation could be moved into the detector response simulatiion (i.e. outside of edep-sim).

[ClarkMcGrew]

"Solved" by making sure that the step limit is working correctly. None of the methods to calculate the energy loss variation inside a segment have proven to be robust.

Work around. For a volume where you need to have redo the Birks calculation, add the StepLimit aux to the logical volume. Set the StepLimit to be smaller than the position resolution so that fluctuations at the boundary of a pad can be handled with out splitting up the charge from a single step.