add logistic decay function

[bstabler]

In addition to flat or linear weighting in pandana, we need logistic weighting. The previous DaySim data tools provided a logistic function for weighting, which was recommended because its shape is more representative of typical behavioral models that use logistic functions.

I approximated the logistic function using a series of flat aggregations in the attached Excel workbook. I did this using a stepwise flat weighting that calculates the buffer many times and then aggregates the result.

buffering_logistic.xlsx