spatial interaction function. Options are "p"
(pareto, power law) or "e" (exponential).
For pareto the interaction is defined as: (1 + alpha * mDistance) ^ (-beta).
For "exponential" the interaction is defined as:
exp(- alpha * mDistance ^ beta).
The alpha parameter is computed from parameters given by the user
(beta and span).
span
distance where the density of probability of the spatial
interaction function equals 0.5.
beta
impedance factor for the spatial interaction function.
limit
maximum distance used to retrieved x points, in
map units.