Deserializes a Block's header, and the header of its Batches.

def _expand_block(cls, block):
        """Deserializes a Block's header, and the header of its Batches.
        """
        cls._parse_header(BlockHeader, block)
        if 'batches' in block:
            block['batches'] = [cls._expand_batch(b) for b in block['batches']]
        return block