SNS-IK velocity solvers should inherit from KDL::ChainIkSolverVel

[MatthewPeterKelly]

It would be great if we made the SNS-IK velocity solvers inherit from the KDL::ChainIkSolverVel class. This would then allow us to pass any of our velocity solvers to the KDL::ChainIkSolverPos_NR position solver, and the KDL::ChainIkSolverPos_NR_JL position solver.