Rename "Root Task"

I've overloaded the word "root" with multiple meanings. One of those meanings is "the daemon or activity task original given to the simulator". All tasks are, or are spawned by, a "root" task in this sense of the word. "root" as a descriptor is correct, but imprecise. In the context of incremental simulation, we have a DAG, and tasks naturally form "branches" of nodes. Extending the plant metaphor, we record the "root" of a task branch - which is an entirely different meaning of the word "root", and an entirely different concept from the task root.

Replace the first meaning of a "root task" with another term (some suggestions below). Since the "root task" terminology is pervasive in the code base and documentation, this will require small touches to lots of locations in the code. Also, since "root task" is overloaded and we're trying to disambiguate those overloaded uses, not every instance of the term should be replaced - only those instances that mean "the original task given to the simulator" should be renamed.

Suggested names include:

• original task
• originator task
• top task
• head task
• anchor task
• base task
• principal task
• prime task
• source task
• initiator task
• starter task
• initial task