Create Sequence objects for each FASTA sequence.

def get_sequences(self, pdb_id = None):
        '''Create Sequence objects for each FASTA sequence.'''
        sequences = {}
        if pdb_id:
            for chain_id, sequence in self.get(pdb_id, {}).iteritems():
                sequences[chain_id] = Sequence.from_sequence(chain_id, sequence)
        else:
            for pdb_id, v in self.iteritems():
                sequences[pdb_id] = {}
                for chain_id, sequence in v.iteritems():
                    sequences[pdb_id][chain_id] = Sequence.from_sequence(chain_id, sequence)
        return sequences