Initialize the observation table.

def _init_table(self):
        """
        Initialize the observation table.
        """
        self.observation_table.sm_vector.append('')
        self.observation_table.smi_vector = list(self.alphabet)
        self.observation_table.em_vector = list(self.alphabet)

        for i in self.observation_table.em_vector:
            self._fill_table_entry('', i)

        for s, e in product(self.observation_table.smi_vector, self.observation_table.em_vector):
            self._fill_table_entry(s, e)