Fix tree visualization for sparse trees

[souravchk25]

The t(s,a,s_prime) matrix is assumed to be always dense regardless of the tree size, however, it is possible that a student initializes by assigning 0 to all triplets and then overwrite the ones which eventually gets hit.

This misstep creates the tree to be extremely sparse even if the tree is very small and visualizable.

visualize_tree() can disregard the 0 values provided.

[zsunberg]

Thanks, @souravchk25 , will fix this for next year.