path.type
Character vector. A user-defined vector that characterizes the cell progression dynamics.
The first element can be either "circular" or "A2Z" or "other". If "circular" the path progression is
assummed to exhibit a circle-like behavior. If "A2Z" the path is assumed to have a well-defined start
and a well-defined end point (e.g. a linear progression). If "other" the progression is assumed to be
arbitrary without an obvious directionality.