Add IMU sensor to simulation

This MR adds the IMU sensor back into the simulation.

Previously, when we were using ROS 1, we were using the hector_gazebo_plugins's implementation of gazebo_ros_imu. This has not been ported over to ROS 2 and so we can no longer use it.

As per this Gazebo tutorial, I have used the gazebo_plugins's implementation of gazebo_ros_imu_sensor instead. This appears to be less featureful, but it seems to work for now. We can look to implement our own IMU sensor (based on hector_gazebo_plugins) at a future date. But given that the IMU sensor is urgently needed, this implementation should be fine for now.

The IMU publishes a sensor_msgs/Imu message to the /imu/out topic. I can't seem to control the out part of the topic, but I can control the /imu part. So if you would like a different topic name, keep that in mind.

Closes #35 (closed).