Meter to measure flux across a boundary or boundaries. If an atom is on one
side of the boundary at one time and on the other side of the boundary the
next time, then the meter counts that as a crossing. Each boundary can have
a coefficient, such that flow into one region through boundaries on either
side can be considered "positive" flux.
If an IntegratorMD is used, flux will be given in terms of crossings per
area per time. Otherwise, flux will be given in terms of crossings per
area per step.