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/dc_motor/headers/dc_motor.h

@@ -0,0 +1,73 @@
+/*----------------------------------------------------------------------------*/
+/* 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 <gazebo/physics/physics.hh>
+#include <gazebo/transport/transport.hh>
+
+#include "simulation/gz_msgs/msgs.h"
+
+/**
+ * \brief Plugin for controlling a joint with a DC motor.
+ *
+ * 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 DC motor to your robot, add the following XML to your
+ * robot model:
+ *
+ *     <plugin name="my_motor" filename="libdc_motor.so">
+ *       <joint>Joint Name</joint>
+ *       <topic>~/my/topic</topic>
+ *       <multiplier>Number</multiplier>
+ *     </plugin>
+ *
+ * - `joint`: Name of the joint this Dc motor is attached to.
+ * - `topic`: Optional. Message type should be gazebo.msgs.Float64.
+ * - `multiplier`: Optional. Defaults to 1.
+ */
+class DCMotor : public gazebo::ModelPlugin {
+ public:
+  /// \brief Load the dc motor and configures 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 magic torque multiplier. torque=multiplier*signal
+  double multiplier;
+
+  /// \brief The joint that this dc motor drives.
+  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 Pointer toe the world update function.
+  gazebo::event::ConnectionPtr updateConn;
+
+  /// \brief The node on which we're advertising.
+  gazebo::transport::NodePtr node;
+
+  /// \brief Subscriber handle.
+  gazebo::transport::SubscriberPtr sub;
+};