Big expansion: working with rebalancers and input attraction chart.

Rebalancers implemented. Algorithm probably still needs work. Also
added a mechanism for importing attraction values from a chart
with values for origin and destination attraction for residential,
employment, and service employment sectors. These are normalized
by the total pop, emp, and svc emp near each station to calculate
relative probabilities of rides originating and ending there.
They are also used (summed) to calculate the overall probability of
generating a ride at any given timestep.