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/potentiometer/headers/potentiometer.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 "simulation/gz_msgs/msgs.h"
+
+/**
+ * \brief Plugin for reading the angle of a joint.
+ *
+ * This plugin publishes the angle of a joint to the topic every
+ * physics update. Supports reading in either radians or degrees.
+ *
+ * To add a potentiometer to your robot, add the following XML to your
+ * robot model:
+ *
+ *     <plugin name="my_pot" filename="libpotentiometer.so">
+ *       <joint>Joint Name</joint>
+ *       <topic>~/my/topic</topic>
+ *       <units>{degrees, radians}</units>
+ *     </plugin>
+ *
+ * - `joint`: Name of the joint this potentiometer is attached to.
+ * - `topic`: Optional. Message will be published as a gazebo.msgs.Float64.
+ *            The default topic name will be the plugin name.
+ *            Recommended topic names are of the form:
+ *              /gazebo/frc/simulator/analog/n
+ *            where n is the analog channel for the pot.
+ * - `multiplier`: Optional.  Amount to multiply output with.
+ *            Useful when a rotary pot returns something other than an angle
+ * - `units`: Optional. Defaults to radians.
+ */
+class Potentiometer : public gazebo::ModelPlugin {
+ public:
+  /// \brief Load the potentiometer and configures it according to the sdf.
+  void Load(gazebo::physics::ModelPtr model, sdf::ElementPtr sdf);
+
+  /// \brief Sends out the potentiometer reading each timestep.
+  void Update(const gazebo::common::UpdateInfo& info);
+
+ private:
+  /// \brief Publish the angle on this topic.
+  std::string topic;
+
+  /// \brief Whether or not this potentiometer measures radians or degrees.
+  bool radians;
+
+  /// \brief A scaling factor to apply to this potentiometer.
+  double multiplier;
+
+  /// \brief The joint that this potentiometer measures
+  gazebo::physics::JointPtr joint;
+
+  /// \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 Publisher handle.
+  gazebo::transport::PublisherPtr pub;
+};