message: ENH: add Matrix::IsSquare() method
diff: @@ -42,6 +42,9 @@ public:
  /// Creates a matrix of the given height and width.
  explicit Matrix(int h, int w) : Operator(h, w) { }
  + /// Returns whether this matrix instanse is a square matrix.
  + bool IsSquare() const { return (height==width); };
  +
  /// Returns reference to a_{ij}.
  virtual double &Elem(int i, int j) = 0;
  