Create Odometer.java

/** A class that creates an Odometer object to track fuel and mileage for a car.
 *  Class contains instance variables to track the miles driven and the fuel 
 *  efficiency in miles per gallon. Class includes mutator methods to reset the
 *  odometer to 0 miles, to set the fuel efficiency and to accept miles driven
 *  for a trip and add it to odometer total. Class includes an accessor method
 *  that returns the number of gallons of gas the car has consumed since the 
 *  odometer was last reset. See test program "OdometerTest.java" that creates
 *  several trips with different fuel efficiencies.
 */