Improve Python API including enabled AsynchronousOut mode (#1593)

* Move producer configuration into configure(...) method of Python API

* Enable output from OpenPose in Python API by making VectorDatum an opaque STL type

* Return empty arrays as None in Python API

* Add type_caster for Array<long long> for datum.poseIds in Python API

* Add ThreadManagerMode enum to Python interface

* Update Python tutorial in line with updated interface and add demo of AsynchronousOut mode

* Remove static_cast of ThreadManagerMode which is no longer needed

* Only set up producer in Python API when SynchronousIn is set

* Fix up memory handling in emplaceAndPop

* Pass AsynchronousOut in async out example

examples/tutorial_api_python/01_body_from_image.py

@@ -60,7 +60,7 @@ try:
     datum = op.Datum()
     imageToProcess = cv2.imread(args[0].image_path)
     datum.cvInputData = imageToProcess
-    opWrapper.emplaceAndPop([datum])
+    opWrapper.emplaceAndPop(op.VectorDatum([datum]))
 
     # Display Image
     print("Body keypoints: \n" + str(datum.poseKeypoints))