Added documentation (robot_motion_tracking_system.md and MotionProcessor.md)

docs/documentation/android/robot_motion_tracking_system.md

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+## Motion Tracking System
+
+---
+
+To capture the user's motion with the sensing devices, we'll need to use a tracking system that calculates the
+path the user makes to be able to determine the user's position and orientation, and therefore whether the movement
+that was made correlates to the provided path.
+This is done by some calculations in the `MotionProcessor` class. To get started, create an instance of the `MotionProcessor` class and call the `processMotion` method with the `MotionData` object as a parameter. This will return a `MotionResult` object that contains the calculated path.
+
+```java
+// create the motion processor
+MotionProcessor motionProcessor = new MotionProcessor();
+```
+
+To start listening for input, one must call the following method:
+
+```java
+// start listening for input
+motionProcessor.startListening();
+```
+
+Calling this function creates a WebSocket server, which the sensing devices can connect to.
+The `MotionProcessor` class will then start listening for a set of messages that start with specified kind of keywords.
+The messages always start with the keyword, separated by a space, and then the data is sent.
+The default data separator is `;`.
+An example of the message format is shown below:
+```java
+// Sending data
+"data accelerationX;accelerationY;accelerationZ;rotationX;rotationY;rotationZ" // all values are floats
+
+// Changing the sample rate
+"sampleRate rate" // rate is an integer
+
+// Calibrating the zero point
+"zero x;y;z" // x, y, z are floats
+``` 
+
+To add a custom message received handler, one can simply call the following method:
+
+```java
+// Add a custom message handler
+motionProcessor.setMotionDataEventHandler((Vector3 vector) -> { ... });
+```
+*Note: The message handler provides a vector as a parameter; this vector is already converted from relative acceleration and rotation.* 
+
+### Error checking
+
+To check whether the made movements correlate with a given path, one must check for their differences.
+This can be done by a few implemented methods:
+ 
+***Get the error of a vector compared to a path***
+```java
+GesturePath path = new GesturePath.Builder()
+                    .addVector(...)
+                    .build();
+                    
+Vector3 referencePoint = new Vector3(...);
+double error = motionProcessor.getError(path, referencePoint);
+```
+
+***Get the average error of a computed path to a `GesturePath`***
+```java
+GesturePath path = new GesturePath.Builder()
+                    .addVector(...)
+                    .build();
+
+double error = motionProcessor.getAverageError(path);
+```
+
+***Get a list of error values from a computed path to a `GesturePath`***
+```java
+GesturePath path = new GesturePath.Builder()
+                    .addVector(...)
+                    .build();
+
+List<Double> errorList = motionProcessor.getErrors(path);
+```
+
+### Example
+
+An example of how to use a motion tracking system is shown below:
+
+```java
+
+// create the motion processor
+MotionProcessor motionProcessor = new MotionProcessor();
+
+// Create a gesture path
+GesturePath.Builder pathBuilder = new GesturePath.Builder();
+
+for ( int i = 0; i < 100; i++ ) 
+    pathBuilder.addVector(new Vector3(i, i, i));
+
+GesturePath path = pathBuilder.build();
+
+// Set the path
+for ( int i = 0; i < 100; i++ ) {
+    motionProcessor.addMotionData(new MotionData(i, i, i, i, i, i));
+}
+
+// Get error values
+List<Double> errorList = motionProcessor.getErrors(path);
+
+// Get average error
+double averageError = motionProcessor.getAverageError(path);
+
+// Now you can do whatever you want with these results.
+```