[sim] Add DoubleSolenoidSim and SolenoidSim classes (#3177)

wpilibc/src/main/native/include/frc/DoubleSolenoid.h

@@ -74,6 +74,20 @@ class DoubleSolenoid : public SolenoidBase,
    */
   void Toggle();
 
+  /**
+   * Get the forward channel.
+   *
+   * @return the forward channel.
+   */
+  int GetFwdChannel() const;
+
+  /**
+   * Get the reverse channel.
+   *
+   * @return the reverse channel.
+   */
+  int GetRevChannel() const;
+
   /**
    * Check if the forward solenoid is blacklisted.
    *