Problem 413. Back to basics 23 - Triangular matrix

Covering some basic topics I haven't seen elsewhere on Cody.

Given an input matrix, return a matrix with all elements above and to the right of the main diagonal set to zero. Example:

input = [ 1 1 1 1; 1 1 1 1; 1 1 1 1; 1 1 1 1]
output = [1 0 0 0; 1 1 0 0; 1 1 1 0; 1 1 1 1]