gravity: Spatial Accessibility via Gravity Method
In chris31415926535/twostepr:

Luo & Wang 2003, DOI:10.1068/b29120

1	gravity(dist_table, supply, demand, dist, gravity_exp, 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'.
`gravity_exp`	The exponent to use for the inverse distance function.
`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.).