Task class not instance should hold sequences.

This addresses one of the remaining "TODO ISO" comments.  Currently each task proxy instance holds a list of all sequences (which embody the graph recurrence expressions) for the task.  The closest next instance of each sequence determines the true next instance of the task when it spawns, but the sequences do not know the task's cycle point (it is passed to them), so this (this list of sequences) should be a class variable, not an instance variable.  (Probably makes little difference in practice as I suppose the instance variables will all reference the same object).

Test battery passes.