Addresses #1166

User - Manager runs local worker on thread - Gen Worker 0

The manager can start an additional worker thread with the worker ID 0.

This worker is, via W["gen_worker"] = True, and filtering options, made to run gens only.

Tentative interface: libE_specs["gen_on_manager"] = True

Internal - Refactor runners.py

Largely refactors utils/runners.py so additional "Runners" for user functions can be more easily developed/extended.

GlobusComputeRunner and ThreadRunner inherit from Runner.

Runner inspects the contents of the input sim/gen specs and instantiates the correct subclass on the left-hand-side:

>>> sim_specs = {... , "globus_compute_endpoint": "1234}
>>> runner = Runner(sim_specs)
>>> runner
<libensemble.utils.runners.GlobusComputeRunner object at 0x1490ec190>
>>> runner = Runner({"sim_f": print})
>>> runner
<libensemble.utils.runners.Runner object at 0x104e024d0>

Internal - _WorkerIndexer class for indexing into worker arrays

Internal - Workers can be assigned gen_worker.