Update kinematics_metrics.cpp

When the group has fewer than 6 DOF, the Jacobian is of the form 6xM and when multiplied by its transpose, forms a 6x6 matrix that is singular and its determinant is always 0 (or NAN if the solver cannot calculate it).

Since calculating the SVD of a Jacobian is a costly operation, I propose to retain the calculation of the Manipulability Index through the determinant for 6 or more DOF (where it produces the correct result), but use the product of the singular values of the Jacobian for fewer DOF.