Update Barracuda to 0.6.1-preview (#3584)

com.unity.ml-agents/CHANGELOG.md

  - Academy.RegisterSideChannel and UnregisterSideChannel methods were added. (#3391)
  - A tutorial on adding custom SideChannels was added (#3391)
  - The stepping logic for the Agent and the Academy has been simplified (#3448)
- - Update Barracuda to 0.6.0-preview
+ - Update Barracuda to 0.6.1-preview
  * The interface for `RayPerceptionSensor.PerceiveStatic()` was changed to take an input class and write to an output class, and the method was renamed to `Perceive()`.
  - The checkpoint file suffix was changed from `.cptk` to `.ckpt` (#3470)
  - The command-line argument used to determine the port that an environment will listen on was changed from `--port` to `--mlagents-port`.

com.unity.ml-agents/package.json

 	"unity": "2018.4",
 	"description": "Add interactivity to your game with Machine Learning Agents trained using Deep Reinforcement Learning.",
 	"dependencies": {
-		"com.unity.barracuda": "0.6.0-preview"
+		"com.unity.barracuda": "0.6.1-preview"
 	}
 }

docs/Migrating.md

 * Replace your calls to `SetActionMask` on your Agent to `DiscreteActionMasker.SetActionMask` in `CollectDiscreteActionMasks`.
 * If you call `RayPerceptionSensor.PerceiveStatic()` manually, add your inputs to a `RayPerceptionInput`. To get the previous float array output,
  iterate through `RayPerceptionOutput.rayOutputs` and call `RayPerceptionOutput.RayOutput.ToFloatArray()`.
-* Re-import all of your `*.NN` files to work with the updated Barracuda package.
 * Replace all calls to `Agent.GetStepCount()` with `Agent.StepCount`.
 * Replace `IFloatProperties` variables with `FloatPropertiesChannel` variables.