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/encoder/headers/encoder.h

@@ -0,0 +1,112 @@
+/*----------------------------------------------------------------------------*/
+/* 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 reading the speed and relative angle of a joint.
+ *
+ * This plugin publishes the angle since last reset and the speed of a
+ * given joint to subtopics of the given topic every physics
+ * update. There is also a control subtopic that takes three commands:
+ * "start", "stop" and "reset":
+ *
+ * - "start": Start counting ticks from the current count.
+ * - "stop":  Stop counting ticks, pauses updates.
+ * - "reset": Set the current angle to zero.
+ *
+ * To add a encoder to your robot, add the following XML to your
+ * robot model:
+ *
+ *     <plugin name="my_encoder" filename="libencoder.so">
+ *       <joint>Joint Name</joint>
+ *       <topic>~/my/topic</topic>
+ *       <units>{degrees, radians}</units>
+ *       <multiplier>Number</multiplier>
+ *     </plugin>
+ *
+ * - `joint`: Name of the joint this encoder is attached to.
+ * - `topic`: Optional. Used as the root for subtopics. `topic`/position
+ * (gazebo.msgs.Float64),
+ *            `topic`/velocity (gazebo.msgs.Float64), `topic`/control
+ * (gazebo.msgs.String)
+ *    The suggested value for topic is of the form
+ *       ~/simulator/encoder/dio/<n>
+ *    where <n> is the number of the first digital input channel
+ *    used to formulate the encoder
+ *
+ * - `units`: Optional. Defaults to radians.
+ * - `multiplier`: Optional. Defaults to 1.
+ *     This can be used to make the simulated encoder
+ *     return a comparable number of ticks to a 'real' encoder
+ *     Useful facts:  A 'degrees' encoder will report 360 ticks/revolution.
+ *     The k4X encoder type can add another multiple of 4 into the mix.
+ */
+class Encoder : public gazebo::ModelPlugin {
+ public:
+  /// \brief Load the encoder and configures it according to the sdf.
+  void Load(gazebo::physics::ModelPtr model, sdf::ElementPtr sdf);
+
+  /// \brief Sends out the encoder reading each timestep.
+  void Update(const gazebo::common::UpdateInfo& info);
+
+ private:
+  /// \brief Root topic for subtopics on this topic.
+  std::string topic;
+
+  /// \brief Whether or not this encoder measures radians or degrees.
+  bool radians;
+
+  /// \brief A factor to mulitply this output by.
+  double multiplier;
+
+  /// \brief Whether or not the encoder has been stopped.
+  bool stopped;
+
+  /// \brief The zero value of the encoder.
+  double zero;
+
+  /// \brief The value the encoder stopped counting at
+  double stop_value;
+
+  /// \brief The joint that this encoder measures
+  gazebo::physics::JointPtr joint;
+
+  /// \brief Callback for handling control data
+  void Callback(const gazebo::msgs::ConstStringPtr& msg);
+
+  /// \brief Gets the current angle, taking into account whether to
+  ///        return radians or degrees.
+  double GetAngle();
+
+  /// \brief Gets the current velocity, taking into account whether to
+  ///        return radians/second or degrees/second.
+  double GetVelocity();
+
+  /// \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 command_sub;
+
+  /// \brief Publisher handles.
+  gazebo::transport::PublisherPtr pos_pub, vel_pub;
+};