First draft of sensor interfaces.

- Force/torque (wrench)
- Orientation
- IMU (very crude approximation)
Add sensor reference frame field.
Add sensor ref frame field and capability queries.
Merge branch 'master' into sensor_interfaces
Scrape orientation interface prototype.
Explicitly initialize IMU sensor handle members.
Remove Eigen dependency from hardware_interface.

- Expose force-torque and IMU sensor data as const pointers to the raw data.

- Client code should wrap raw data however they prefer.
Merge branch 'master' into sensor_interfaces
Merge branch 'master' into sensor_interfaces
Merge branch 'master' into sensor_interfaces
Update sensor interfaces implementation.

- Use resource managing classes introduced in recent hardware interface rework.

- Conform to unified public API.
Merge branch 'master' into sensor_interfaces
Add missing brace.
Merge branch 'master' into sensor_interfaces
Revert "Fixed PLUGINLIB_DECLARE_CLASS deprecated errors"

This reverts commit cd9aba265a380bafebb70d63081405d857e9380d.
Merge branch 'fuerte_backport' into sensor_interfaces
Add default constructors to sensor handles.
Unit test sensor interfaces.
Merge commit '8054913' into hardware_interface_sensors
Merge commit '9bf48bd' into hardware_interface_sensors