Define the Matrix class and extend the Grid class.

Complete the implementation of the following methods:

$\_\_$init$\_\_$

$\_\_$add$\_\_,$ adds two matrices together.

Returns the sum of self and other.

$\_\_$sub$\_\_,$ subtracts two matrices.

Returns the difference of self and other.

$\_\_$mul$\_\_,$ multiplies two matrices together.

Returns the product of self and other.

If two matrices cannot be multiplied, raise an Exception.