Add default values for b and zeta in RamseteController (#2145)

Values b=2.0 and zeta=0.7 have been well-tested to produce desirable results.

wpilibj/src/main/java/edu/wpi/first/wpilibj/controller/RamseteController.java

@@ -63,6 +63,15 @@ public class RamseteController {
     m_zeta = zeta;
   }
 
+  /**
+   * Construct a Ramsete unicycle controller. The default arguments for
+   * b and zeta of 2.0 and 0.7 have been well-tested to produce desireable
+   * results.
+   */
+  public RamseteController() {
+    this(2.0, 0.7);
+  }
+
   /**
    * Returns true if the pose error is within tolerance of the reference.
    */