Service to find object distance from camera

[mdepp]

Make a node that exposes a service to convert 2D image coordinates to 3D coordinates in some coordinate frame. This can be done using the data from the rgbd-camera and the image_geometry package.

Note that while the simulated rgbd camera is very reliable, the real camera will not be, so it may be necessary to sample several points to get one with a defined depth. It may also be beneficial to use the average depth from several adjacent points to smooth out any errors.

This is needed for the prospecting task.