PenaltySP_FE is a subclass of FE_Element used to enforce a
single point constraint. It does this by adding \alpha to the
tangent and \alpha * (U_s - U_t) to the residual at the locations
corresponding to the constrained degree-of-freedom specified by the
SP_Constraint, where U_s is the specified value of the constraint
and U_t the current trial displacement at the node corresponding to
the constraint.