Mechanism for grouped Task execution

[andrewkaufman]

Clumping is an organizational thing, doing several tasks at once for efficiency. It's quite dependent on what each ExecutableNode is doing, which seems to suggest it should be a per-node setting rather than a global one.

The Despatcher needs to implement clumping internally by making "super tasks" which contain each task in the clump - then anything that depended on one of those tasks now depends on the "super task" instead.