Whether a policy is done, or still working on the goal in hand.

Mirrors tbp/monty/frameworks/models/motor_policies.py::PolicyStatus. Monty's values are the strings "ready" and "in_progress"; here they are atoms, which is what the rest of this port does with small closed sets, and the strings appear only on a wire — as with Monty.Cmp.Message.sender_type. The enum members are values rather than functions, so the atoms are the whole of this module (docs/monty-differences.md §2.8).