Arrange a choice of sensors for ElevatorAttendant so we can try them

When we get our 90 minutes with the Robot on Feb 22, we need to be able to test the navx ability to predict the height of the elevator.

Later on, we may have sonar and/or encoders that could do something similar. Make sure that everything will plug-in okay so we can easily switch out the sensor we are using for ElevatorAttendant.

Start by locating ElevatorAttendant and the current navx height-prediction path. Determine how a sensor choice could support navx now and sonar or encoders later; done means the sensor can be switched without changing ElevatorAttendant's surrounding behavior.

Written by the indexing model from the issue text.