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/pneumatic_piston/headers/pneumatic_piston.h

@@ -0,0 +1,102 @@
+/*----------------------------------------------------------------------------*/
+/* 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 joint with a pneumatic piston.
+ *
+ * This plugin subscribes to topics to get the solenoid state for a piston
+ *    It needs one signal for the forward signal.  For a double solenoid,
+ *    a second signal can be sent.
+ *    Each signal is a boolean.
+ *
+ * Every physics update the joint's torque is set to reflect the
+ * signal, using the configured force
+ *
+ * To add a pneumatic piston to your robot, add the following XML to
+ * your robot model:
+ *
+ *     <plugin name="my_piston" filename="libpneumatic_piston.so">
+ *       <joint>Joint Name</joint>
+ *       <topic>/gazebo/frc/simulator/pneumatics/1/1</topic>
+ *       <reverse-topic>/gazebo/frc/simulator/pneumatics/1/2</reverse-topic>
+ *       <direction>{forward, reversed}</direction>
+ *       <forward-force>Number</forward-force>
+ *       <reverse-force>Number</reverse-force>
+ *     </plugin>
+ *
+ * - `joint`: Name of the joint this piston is attached to.
+ * - `topic`: Optional. Forward Solenoid signal name. type gazebo.msgs.Bool.
+ *            If not given, the name given for the plugin will be used.
+ *            A pattern of /gazebo/frc/simulator/pneumatics/1/n is good.
+ *            The first number represents the PCM module.  Only 1 is supported.
+ *            The second number represents the channel on the PCM.
+ * - `topic-reverse`: Optional. If given, represents the reverse channel.
+ *            Message type should be gazebo.msgs.Bool.
+ * - `direction`: Optional. Defaults to forward. Reversed if the piston
+ *                pushes in the opposite direction of the joint axis.
+ * - `forward-force`: Force to apply in the forward direction.
+ * - `reverse-force`: Force to apply in the reverse direction.
+ *                    For a single solenoid, you would expect '0',
+ *                    but we allow model builders to provide a value.
+ *
+ */
+class PneumaticPiston : public gazebo::ModelPlugin {
+ public:
+  /// \brief Load the pneumatic piston and configures it according to the sdf.
+  void Load(gazebo::physics::ModelPtr model, sdf::ElementPtr sdf);
+
+  /// \brief Updat the force the piston applies on the joint.
+  void Update(const gazebo::common::UpdateInfo& info);
+
+ private:
+  /// \brief Topic to read forward control signal from.
+  std::string topic;
+
+  /// \brief Topic to read reverse control signal from.
+  std::string reverse_topic;
+
+  /// \brief Whether the solenoid to open forward is on
+  bool forward_signal;
+
+  /// \brief Whether the solenoid to open in reverse is on
+  bool reverse_signal;
+
+  /// \brief The magic force multipliers for each direction.
+  double forward_force, reverse_force;
+
+  /// \brief The joint that this pneumatic piston actuates.
+  gazebo::physics::JointPtr joint;
+
+  /// \brief Callback for receiving msgs and updating the solenoid state
+  void ForwardCallback(const gazebo::msgs::ConstBoolPtr& msg);
+
+  /// \brief Callback for receiving msgs and updating the reverse solenoid state
+  void ReverseCallback(const gazebo::msgs::ConstBoolPtr& msg);
+
+  /// \brief The model to which this is attached.
+  gazebo::physics::ModelPtr model;
+
+  /// \brief Pointer to 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;
+
+  /// \brief Subscriber handle for reverse topic
+  gazebo::transport::SubscriberPtr sub_reverse;
+};