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/clockTest/cpp/clockTest.cpp

@@ -0,0 +1,75 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 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 <simulation/gz_msgs/msgs.h>
+
+#include <gazebo/gazebo.hh>
+#include <gazebo/gazebo_client.hh>
+#include <gazebo/msgs/msgs.hh>
+#include <gazebo/sensors/sensors.hh>
+#include <gazebo/transport/transport.hh>
+
+#include "gtest/gtest.h"
+
+using namespace testing;
+
+static char* library;
+static char* world_sdf;
+static double latest_time;
+
+void cb(gazebo::msgs::ConstFloat64Ptr& msg) { latest_time = msg->data(); }
+
+TEST(ClockTests, test_clock) {
+  gazebo::physics::WorldPtr world;
+
+  ASSERT_TRUE(library);
+  ASSERT_TRUE(!access(library, X_OK | R_OK));
+  ASSERT_TRUE(world_sdf);
+  ASSERT_TRUE(!access(world_sdf, R_OK));
+
+  gazebo::addPlugin(library);
+  ASSERT_TRUE(gazebo::setupServer());
+
+  world = gazebo::loadWorld(world_sdf);
+  ASSERT_TRUE(world != NULL);
+
+  ASSERT_TRUE(gazebo::client::setup());
+
+  gazebo::transport::NodePtr node(new gazebo::transport::Node());
+  node->Init();
+
+  gazebo::transport::SubscriberPtr sub =
+      node->Subscribe("/gazebo/frc/time", cb);
+
+  gazebo::sensors::run_once(true);
+  gazebo::sensors::run_threads();
+  gazebo::common::Time::MSleep(1);
+
+  double start = latest_time;
+
+  for (unsigned int i = 0; i < 1000; ++i) {
+    gazebo::runWorld(world, 1);
+    gazebo::sensors::run_once();
+    gazebo::common::Time::MSleep(1);
+  }
+
+  double end = latest_time;
+
+  ASSERT_TRUE(end > start);
+  ASSERT_TRUE(gazebo::client::shutdown());
+  ASSERT_TRUE(gazebo::shutdown());
+}
+
+int main(int argc, char** argv) {
+  testing::InitGoogleTest(&argc, argv);
+
+  if (argc >= 1) library = argv[1];
+
+  if (argc >= 2) world_sdf = argv[2];
+
+  return RUN_ALL_TESTS();
+}

simulation/frc_gazebo_plugins/src/clockTest/world/clockTest.world

@@ -0,0 +1,17 @@
+<?xml version="1.0" ?>
+<sdf version="1.5">
+    <world name="default">
+        <scene>
+            <ambient>0.2 0.2 0.2 1</ambient>
+            <background>1 1 1 1</background>
+            <shadows>false</shadows>
+            <grid>false</grid>
+            <origin_visual>false</origin_visual>
+        </scene>
+        <model name='Clock'>
+            <plugin name='clock' filename='libclock.so'>
+                <topic>/gazebo/frc/time</topic>
+            </plugin>
+        </model>
+    </world>
+</sdf>