removed method getElementsToVector()
removed method operator* (for vector)
made methods operator*(for Matrix), operator+(for Matrix),
operator-(unary), isEmpty(), isNull(), isSquare(), isZeroMatrix(),
operator==(for Matrix) const
added definition for method operator-(for Matrix) and made it const
renamed case ZeroMatrixAllCases at test MatrixUnaryOperatorMinus to
EmptyMatrixAllCases
added case AllCases at test MatrixHaveSameSize a friend to class
added case AllCases to test MatrixHaveSameSize
added case Matrix3x3PlusMatrix3x3 to test MatrixOperatorPlus
added case NullAndEmptyMatrices to test MatrixOperatorPlus
added case SizesAreNotEqual to test MatrixOperatorPlus
added cases Matrix3x3MinusMatrix3x3, NullAndEmptyMatrices
and SizesAreNotEqual to test MatrixOperatorMinus
cleaned up main.cpp