suggested function for swaRm

[miakent]

It would be useful to have a function that enables the user to define a point, a line and/or a polygon (x, y coordinates) in the environment and calculate each individual's distance to this fixed area at each time stamp. It's a measure I find I constantly need, especially in experiments with foraging patches. It could enable me to look at the order in which individuals arrive at/leave a food patch, how long each individual remains within the patch etc, which can subsequently help identify key individuals driving group movement.