Tidy up iOS sensor encoders

[janm399]

Complete the sensor submission code, integrate with the server code:

• Implement ContinuousSensorDataArray.encode(mutating:) method,
• Submit data to the server in LiveSessionController.deviceSession(session:sensorDataReceivedFrom:atDeviceTime:data:)
• Verify that the accelerometer data encoding in encode_lift_accelerometer_data matches the Pebble code (focus on memory layout)
• Check that that the multi packet is decoded properly on the server (raise another issue if not)