matrices-lists1: add simple traversal

[David Leoni]

https://en.softpython.org/matrices-lists/matrices-lists1-sol.html

Surprisingly, a simple iteration that prints the cells is missing, maybe because something similar is already in for7-nested (see also #58 ) but a reminder is not a bad idea.

Question: in the iteration should we print in the same order as we see with print(mat[i][j], end=' ') (but the end= might be slighlty confusing) or in vertical (which on the other hand might suggest some sort of column traversing)?