Added variance stats
diagnostic: add tracked arg, plot track lifetimes instead of max id, modified plotting, use framenumbers
Added steps arg to update(), step over missed frames
Save assignment cost matrices
Increased cost of unassigned tracks/dets to 10 (was 1)

When stepping multiple frames, costs got higher
Improved initial error estimates
If 'oldnsap' exists, reanalyze that
Added plotassignmentcosts()