Implementation of initial, once, and final tasks.

Try running dev/suites/final-tasks-etc (NOT in graph view yet!)

NOTE: (a) suite graphing has not been updated so is broken; and (b) much
clean-up and code clarification to do, from graph parsing through task
proxy and trigger definition; and check that all uses of these new task
types work as expected with other types and with each other.

**Not ready for merging yet**