Added gapEvolutionTolerance parameter that controls what growth of the gap is...

Added gapEvolutionTolerance parameter that controls what growth of the gap is allowed. If 0.0, no growth is permitted at all (once a pixel is labeled as non-gap, it will always remain non-gap in the future frames); if negative, tolerance is unlimited (any pixel can be labeled as gap regardless of its history); if >0, a pixel can be labeled as gap only in the first frame, or if sufficiently close to a gap in the previous frame.