Add a comparator for joint configurations.

The comparator uses the distance to a reference configuration to compare
two configurations.
Add conversion functions.

These functions convert between joint configurations, represented as
either KDL::JntArray or std::vector.
Apply the conversion function to the seed state.
Rename the variable returned from the kinematics.
Use the conversion function on the IK solutions.
Return the solution closest to the seed state.

Use the configuration comparator to first sort the found solutions based
on their distance to the seed and then easily return the closest
solution.