Simple sampling implementation for cartesian points.

This PR will have to be updated one #33 is accepted.  Also, this PR conflicts with #25.  This PR is simply to address a pressing need to perform sampling on Cartesian points.
It is in the [robot_model](https://github.com/ros-industrial-consortium/descartes/blob/hydro-devel/descartes_core/include/descartes_core/robot_model.h#L96) interface.