coordinates_of_points_in_subcollection function

This is an auxiliery function. It take the coordinates of points, ids of subset of points, and number of coordinate, and return a sorted vector of the given coodrinate in the considered points. For instance, given the collection of points: 1 2 3 4 5 6 7 8 9 and which_subset = 2,3 and number_of_coordinate = 2 the procedure below will return the vector [2,5,8].