PenaltyMP_FE is a subclass of FE_Element used to enforce a
multi point constraint, of the form \U_c = \C_{cr} \U_r, where \U_c are
the constrained degrees-of-freedom at the constrained node, \U_r are
the retained degrees-of-freedom at the retained node and \C_{cr} a
matrix defining the relationship between these degrees-of-freedom.
To enforce the constraint a matrix \alpha \C^T \C is added to the
tangent for the degrees-of-freedom [\U_c \U_r], where \C = [-\I
\C_{cr}]. Nothing is added to the residual.
virtual const