message: Adding MatrixConstantCoefficient
  This adds a MatrixConstantCoefficient analogous to the
  ConstantCoefficient and VectorConstantCoefficient classes.
diff: @@ -385,6 +385,18 @@ public:
  virtual ~MatrixCoefficient() { }
  };
  +class MatrixConstantCoefficient : public MatrixCoefficient
  +{
  +private:
  + DenseMatrix mat;
  +public:
  + MatrixConstantCoefficient(const DenseMatrix &m)
  + : MatrixCoefficient(m.Size()), mat(m) { }
  + using MatrixCoefficient::Eval;
  + virtual void Eval(DenseMatrix &M, ElementTransformation &T,
  + const IntegrationPoint &ip) { M = mat; }
  +};
  +
  class MatrixFunctionCoefficient : public MatrixCoefficient
  {
  private:
  