na364/PIGLE: Mex Interactions and Form Factor

In this tag, the interactions are written in C mex, and connection list is implemented in mex as well. Furthermore, a digitized form-factor can be assigned to each population: both for the centre of mass, and/or for the scattering centres of the particles in each population. Various help functions are introduced, for example, to compute dipole-dipole Khun-Lau based interaction.