Rework how contacts are made into spots in the TrackMate detector.
 * Spot radius represents the spot size (equivalent area of circle for
   2D, and volume of equivalent sphere for 3D).
 * Quality stores the total contact intensity in object.
 * The threshold paramter operates on the contact size (in pixels).

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>