Fix for some inefficient behavior in contact spread module. When the simulator decides that a contact should go "d" km, it scans for a recipient best matching that distance within a 2xd radius around the infection source. If it doesn't find one, it falls back on a scan of the area outside that circle. But in the case when d is very small (a very short-distance contact is desired), this doesn't work well because there may be no, or very few, units within 2xd km. This results in a fallback scan for the sake of a very short-distance contact, which is inefficient. Now the code includes a minimum radius for the initial scan. Experimentally, a circle that is expected to include 3 dozen units (based on a guesstimate of population density) seems to be a good choice.