Write a function called even_index that takes a matrix, M, as input argument and returns a matrix that contains only those elements of M that are in even rows and columns. In other words, it would return the elements of M at indices (2,2), (2,4), (2,6), , (4,2), (4,4), (4,6), , etc. Note that both the row and the column of an element must be even to be included in the output. The following would not be returned: (1,1), (2,1), (1,2) because either the row or the column or both are odd. As an example, if M were a 5-by-8 matrix, then the output must be 2-by-4 because the function omits rows 1, 3 and 5 of M and it also omits columns 1, 3, 5, and 7 of M.