Implement the probability_path method which takes a string, a mapping from letter to index and a transition matrix.
The function should return the probability of the given path.
>>> probability_path('BABAA', {'B': 1, 'A': 0}, [[0.24439433296892832, 0.7556056670310717], [0.24241542404217956, 0.7575845759578205]]) 0.005425963151164815 >>> probability_path('AABABABAAA', {'B': 1, 'A': 0}, [[0.3981195623715506, 0.6018804376284493], [0.3924198144356077, 0.6075801855643923]]) 0.00041571340299273414