tsfca: Calculate Two-Step Floating Catchment Areas (2SFCA)
In chris31415926535/twostepr:

Calculate the accessibility index using the two-step floating catchment area (2SFCA) method.

1	tsfca(dist_table, supply, demand, dist, dist_threshold, supply_vol, demand_vol)

`dist_table`	A data frame containing an origin-destination (OD) table, with additional columns for supply and demand volumes.
`supply`	The name of the column in 'dist_table' that uniquely identifies supply points (hospitals, physicians, etc.).
`demand`	The name of the column in 'dist_table' that uniquely identifies demand points (census tracts, residences, etc.).
`dist`	The name of the column in 'dist_table' that contains the distance between the 'supply' and the 'demand'.
`dist_threshold`	The distance threshold defining catchment areas.
`supply_vol`	The name of the column in 'dist_table' that contains supply volumes (# of hospital beds, # of physicians, etc.).
`demand_vol`	The name of the column in 'dist_table' that contains demand volumes (population counts, etc.).