Bring back the gazebo plugins (#1063)

The models and meshes are not included. We will need
to find an alternate way to reintegrate these and use them.

* Add simulation/gz_msgs back, and build with Gradle.

* Add back in the frc simulation plugins for gazebo.

* Add a new shared library, halsim_gazebo.

This library will become the interface between the
HAL sim layer and gazebo.

* Preserve the first channel number used in created Encoders in the Sim MockData.

This allows us to use the DIO channel number to connect with simulated encoders.

* Have the HAL Simulator set the reverse direction on creation.

This enables a simulator to be aware of the direction.

* Add a drive_motor plugin.

This is a bit of a 'magic' motor, which allows us to build robot
models that drive in a more realistic fashion.  It does this
by apply forces directly to the chassis, rather than relying on
the complex motion dynamics of a driven wheel.

This in turn allows the model to reduce wheel friction,
reducing scrub, and allowing for a more natural driving experience.

simulation/frc_gazebo_plugins/src/servo/headers/servo.h

@@ -0,0 +1,70 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2016-2018 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <string>
+
+#include <gazebo/gazebo.hh>
+
+#include "simulation/gz_msgs/msgs.h"
+
+/**
+ * \brief Plugin for controlling a servo.
+ *
+ * This plugin subscribes to a topic to get a signal in the range
+ * [-1,1]. Every physics update the joint's torque is set as
+ * multiplier*signal.
+ *
+ * To add a servo to your robot, add the following XML to your robot
+ * model:
+ *
+ *     <plugin name="my_servo" filename="libservo.so">
+ *       <joint>Joint Name</joint>
+ *       <topic>/gzebo/frc/simulator/pwm/1</topic>
+ *       <zero_position>0</zero_position>
+ *     </plugin>
+ *
+ * - `link`: Name of the link the servo is attached to.
+ * - `topic`: Optional. Message type should be gazebo.msgs.Float64.
+ */
+class Servo : public gazebo::ModelPlugin {
+ public:
+  /// \brief load the servo and configure it according to the sdf
+  void Load(gazebo::physics::ModelPtr model, sdf::ElementPtr sdf);
+
+  /// \brief Update the torque on the joint from the dc motor each timestep.
+  void Update(const gazebo::common::UpdateInfo& info);
+
+ private:
+  /// \brief Topic to read control signal from.
+  std::string topic;
+
+  /// \brief the pwm signal limited to the range [-1,1]
+  double signal;
+
+  /// \brief the torque of the motor in kg/cm
+  double torque;
+
+  /// \brief the joint that this servo moves
+  gazebo::physics::JointPtr joint;
+
+  /// \brief Callback for receiving msgs and storing the signal
+  void Callback(const gazebo::msgs::ConstFloat64Ptr& msg);
+
+  /// \brief The model to which this is attached
+  gazebo::physics::ModelPtr model;
+
+  /// \brief The pointer to the world update function
+  gazebo::event::ConnectionPtr updateConn;
+
+  /// \brief The node on which we're advertising torque
+  gazebo::transport::NodePtr node;
+
+  /// \brief The subscriber for the PWM signal
+  gazebo::transport::SubscriberPtr sub;
+};