Returns symbol from symbols list representing the state for taxon `i' and character `j'.
The normal situation in which there is only one state with no uncertainty or polymorphism is represented by `k' = 0. If there are multiple states, specify a number in the range [0..n) where n is the number of states returned by the GetNumStates function. Use the IsPolymorphic function to determine whether the multiple states correspond to uncertainty in state assignment or polymorphism in the taxon. Assumes `symbols' is non-NULL.