message: Adding methods to remove entries from piecewise coefficients
diff: @@ -162,6 +162,10 @@ public:
  void UpdateCoefficient(int attr, Coefficient & coef)
  { pieces[attr] = &coef; }
  + /// Remove a single Coefficient for a particular attribute
  + void ZeroCoefficient(int attr)
  + { pieces.erase(attr); }
  +
  /// Evaluate the coefficient.
  virtual double Eval(ElementTransformation &T,
  const IntegrationPoint &ip);
  @@ -492,6 +496,10 @@ public:
  /// Replace a single Coefficient for a particular attribute
  void UpdateCoefficient(int attr, VectorCoefficient & coef);
  + /// Remove a single VectorCoefficient for a particular attribute
  + void ZeroCoefficient(int attr)
  + { pieces.erase(attr); }
  +
  /// Evaluate the coefficient.
  virtual void Eval(Vector &V, ElementTransformation &T,
  const IntegrationPoint &ip);
  @@ -924,6 +932,10 @@ public:
  /// Replace a single coefficient for a particular attribute
  void UpdateCoefficient(int attr, MatrixCoefficient & coef);
  + /// Remove a single MatrixCoefficient for a particular attribute
  + void ZeroCoefficient(int attr)
  + { pieces.erase(attr); }
  +
  /// Evaluate the coefficient.
  virtual void Eval(DenseMatrix &K, ElementTransformation &T,
  const IntegrationPoint &ip);
  