Task 2: Submatrix Sum
Submitted by: Jorg Sommrey

You are given a NxM matrix A of integers.

Write a script to construct a (N-1)x(M-1) matrix B 
having elements that are the sum over the 2x2 submatrices of A,

b[i,k] = a[i,k] + a[i,k+1] + a[i+1,k] + a[i+1,k+1]


Example 1

Input: $a = [
              [1,  2,  3,  4],
              [5,  6,  7,  8],
              [9, 10, 11, 12]
            ]

Output: $b = [
               [14, 18, 22],
               [30, 34, 38]
             ]