Impl is_close(packet,point)

Trilateration occurs with 3 points.  However, if >3 sensors record a shot
the packets from the remaining sensors should still be attached to the shot.
This is done by checking if the remaining sensor packets lie 'close' to the
point of fire.  Radial distance is used to check 'closeness'.

A global variable single_point_match_error controls how large the range for
'closeness' is.