Clarify matrix-vector mapping in the HiSSE tutorial

[davidcerny]

This is just a small addition to the HiSSE tutorial inspired by the question we received in issue #660: without specifying whether we fill the rate matrix by column or by row, it's not clear which elements of the transition_rates vector correspond to which rates.

[brpetrucci]

Looks good! Maybe would be good to have a comment like this under the hidden rate definitions instead, to clarify which rate transitions are e.g. q0A->1A vs. q0A->0B etc? But this change is useful regardless!