DXWorkflow supports new stage_instance_types API.

* DXExecutable._inst_type_to_sys_reqs now requires either "dx_insttype"
  string or {"entrypoint": "dx_insttype"} map.
* stage_instance_types may be {"stage": "dx_insttype"} or {"stage":
  {"entrypoint": "dx_insttype"}}
* Fixed a bug in retrieval of project when running a workflow (oops).
* Handling of the instance_type arg is moved to DXExecutable since it
  can be interpreted in the same way across all executables.
* Fixed a bug where DXWorkflow.run didn't return the analysis handler.
* Add tests.