Write a function named custom_blocks that takes an n-by-m
matrix as an input argument (the function does not have to check the format
of the input) and returns a 2n-by-2m matrix as an output argument. The upper
left n-by-m sub matrix of the output matrix is the same as the input matrix.
The elements of the upper right n-by-m sub matrix are twice as large as
the corresponding elements of the input matrix. Similarly, the lower left submatrix
is the input matrix multiplied by three and the lower right n-by-m submatrix
is four times the input matrix. For example, here is an example run:
>> custom_blocks([1:3;3:-1:1])
ans =
1 2 3 2 4 6
3 2 1 6 4 2
3 6 9 4 8 12
9 6 3 12 8 4