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/cpp/encoder.cpp

@@ -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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "encoder.h"
+
+#include <boost/algorithm/string/replace.hpp>
+
+GZ_REGISTER_MODEL_PLUGIN(Encoder)
+
+void Encoder::Load(gazebo::physics::ModelPtr model, sdf::ElementPtr sdf) {
+  this->model = model;
+
+  // Parse SDF properties
+  joint = model->GetJoint(sdf->Get<std::string>("joint"));
+  if (sdf->HasElement("topic")) {
+    topic = sdf->Get<std::string>("topic");
+  } else {
+    topic = "~/" + sdf->GetAttribute("name")->GetAsString();
+  }
+
+  if (sdf->HasElement("units")) {
+    radians = sdf->Get<std::string>("units") != "degrees";
+  } else {
+    radians = true;
+  }
+  multiplier = 1.0;
+  zero = GetAngle();
+  stopped = true;
+  stop_value = 0;
+
+  if (sdf->HasElement("multiplier"))
+    multiplier = sdf->Get<double>("multiplier");
+
+  gzmsg << "Initializing encoder: " << topic << " joint=" << joint->GetName()
+        << " radians=" << radians << " multiplier=" << multiplier << std::endl;
+
+  // Connect to Gazebo transport for messaging
+  std::string scoped_name =
+      model->GetWorld()->GetName() + "::" + model->GetScopedName();
+  boost::replace_all(scoped_name, "::", "/");
+  node = gazebo::transport::NodePtr(new gazebo::transport::Node());
+  node->Init(scoped_name);
+  command_sub = node->Subscribe(topic + "/control", &Encoder::Callback, this);
+  pos_pub = node->Advertise<gazebo::msgs::Float64>(topic + "/position");
+  vel_pub = node->Advertise<gazebo::msgs::Float64>(topic + "/velocity");
+
+  // Connect to the world update event.
+  // This will trigger the Update function every Gazebo iteration
+  updateConn = gazebo::event::Events::ConnectWorldUpdateBegin(
+      boost::bind(&Encoder::Update, this, _1));
+}
+
+void Encoder::Update(const gazebo::common::UpdateInfo& info) {
+  gazebo::msgs::Float64 pos_msg, vel_msg;
+  if (stopped) {
+    pos_msg.set_data(stop_value * multiplier);
+    pos_pub->Publish(pos_msg);
+    vel_msg.set_data(0);
+    vel_pub->Publish(vel_msg);
+  } else {
+    pos_msg.set_data((GetAngle() - zero) * multiplier);
+    pos_pub->Publish(pos_msg);
+    vel_msg.set_data(GetVelocity() * multiplier);
+    vel_pub->Publish(vel_msg);
+  }
+}
+
+void Encoder::Callback(const gazebo::msgs::ConstStringPtr& msg) {
+  std::string command = msg->data();
+  if (command == "reset") {
+    zero = GetAngle();
+  } else if (command == "start") {
+    stopped = false;
+    zero = (GetAngle() - stop_value);
+  } else if (command == "stop") {
+    stopped = true;
+    stop_value = GetAngle();
+  } else {
+    gzerr << "WARNING: Encoder got unknown command '" << command << "'."
+          << std::endl;
+  }
+}
+
+double Encoder::GetAngle() {
+  if (radians) {
+    return joint->GetAngle(0).Radian();
+  } else {
+    return joint->GetAngle(0).Degree();
+  }
+}
+
+double Encoder::GetVelocity() {
+  if (radians) {
+    return joint->GetVelocity(0);
+  } else {
+    return joint->GetVelocity(0) * (180.0 / M_PI);
+  }
+}