BUG/TST: Points were not removed from memory.

When a Track was put in memory and then "found" again, it remained in memory (mem_set). Therefore, two Points could link the same track in the same frame -- one via the point in memory, and one via the new point at which the Track was found again.

See test case for an example of the problem. The PointND(3, [4, 1]) *should* start a new Track, but before this change it was attached to the same track, in the same level, as Point(3, [2, 1]). This is obviously not OK.

Also, some particles were not being added to memory that should have been. I believe this is the minimal change that treats memory correctly.