Requires specification of two target lists. Calculates the average distance from particles in list 1 to the closest member of list 2.
mean_closest_distance <output file name> <timescheme>
<timescheme> selects which times are averaged over. If timescheme = -1, loop over all times. If timescheme is zero or positive, only use one time per block, with the value setting the time index offset from the beginning of the block.