When parsing a schema with message tags that don't specify the `blockLength` attribute, I get this error: ``` ... ...\lib\site-packages\sbe\__init__.py", line 1088, in _parse_schema blockLength=int(attrs['blockLength']), KeyError: 'blockLength' ``` It looks like the parser depends on this attribute being there, however [SBE specifies](http://fixprotocol.io/2016/sbe/sbe.xsd) that blockLength is optional: ``` <xs:attribute name="blockLength" type="xs:nonNegativeInteger" use="optional"> ```