Computation of the pdf for the "rate of approach" #333
Description
The current implementation of the pair-distribution function (PDF) may only be used to evaluate the "distance" probability between individuals. However, in Cordes et al. (2023) and Karamousas et al. (2014), the PDF for the rate of approach is also investigated. This is the one leading to the famous "power-law" model from Karamousas et al.