Post Lab 30
  Write a function called sum_major_diagonal(a_matrix), which
  takes a square 2-d list as a parameter.  Your function should return the sum
  of the elements along the major diagonal.
Two-dimensional Lists
  Write a function called sum_major_diagonal(a_matrix), which
  takes a square 2-d list as a parameter.  Your function should return the sum
  of the elements along the major diagonal.