added gazebo plugins for d435i's IMU

[m-tartari]

The D435i camera publishes IMU data on 2 different topics (/camera/accel/sample and /camera/gyro/sample) which publish from different frames (/camera_accel_optical_frame and /camera_gyro_optical_frame respectively)

• /camera/accel/sample publishes sensor_msgs/Imu messages containing:

header: 
  seq: xxxxx
  stamp: 
    secs: xxxxx
    nsecs: xxxxx
  frame_id: /camera_accel_optical_frame
orientation: 
  x: 0.0
  y: 0.0
  z: 0.0
  w: 0.0
orientation_covariance: [-1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
angular_velocity: 
  x: 0.0
  y: 0.0
  z: 0.0
angular_velocity_covariance: [0.1, 0.0, 0.0, 0.0, 0.1, 0.0, 0.0, 0.0, 0.1]
linear_acceleration: 
  x: x.x
  y: x.x
  z: x.x
linear_acceleration_covariance: [x.x, 0.0, 0.0, 0.0, x.x, 0.0, 0.0, 0.0, x.x]

• /camera/gyro/sample publishes sensor_msgs/Imu messages containing:

header: 
  seq: xxxxx
  stamp: 
    secs: xxxxx
    nsecs: xxxxx
  frame_id: /camera_gyro_optical_frame
orientation: 
  x: 0.0
  y: 0.0
  z: 0.0
  w: 0.0
orientation_covariance: [-1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
angular_velocity: 
  x: x.x
  y: x.x
  z: x.x
angular_velocity_covariance: [x.x, 0.0, 0.0, 0.0, x.x, 0.0, 0.0, 0.0, x.x]
linear_acceleration:
  x: 0.0
  y: 0.0
  z: 0.0
linear_acceleration_covariance: [0.1, 0.0, 0.0, 0.0, 0.1, 0.0, 0.0, 0.0, 0.1] 

I've made 2 independent gazebo plug-ins that are stripped-down versions of standard gazebo imu (gazebo_ros_imu_sensor).

I've added in README.md the instructions to use them in the xacro files.

[m-tartari]

I've removed the usage section for the README.md, I've placed replaced it with the following:

" The plugins are needed for the URDF models of realsense cameras that will soon be available at pal-robotics-forks/realsense.
A working model using this plugin can currently be found at m-tartari/realsense_gazebo_description. "

It's meant to be a temporary statement (for as long as the xacro files on pal-robotics-forks/realsense are not updated). Feel free to further comment and/or request changes.