fsa_times_fsa does not work with MultiGraphs

[braraki]

there are two calls to fsa.g[u][v]['guard'] and fsa.g[u][v]['input'] --- in the case of a MultiGraph, these must be fsa.g[u][v][key]['guard'] etc, where 'key' is an extra index that MultiGraphs have. I've implemented this in my branch.