Mel lain new topic ids

Change to 

def GetTopicModel(self, model = None, args = messages_pb2.GetTopicModelArgs()):
  if (model is not None)
    args.model_name = model.name()

The idea is that the change of python interface should be as backwards compatible as possible, and the interface should be as convenient to use as possible. Keep the "model" parameter for simple usecase when user wants to simply retrieve a model, and keep the "args" parameter for all advanced scenarios.

The same comment applies to GetThetMatrix