Add rotation attribute in ALTO output

[kermitt2]

For some reason, the rotation attribute which was present in pdf2xml and which is still computed, is not outputted in the ALTO file presently. If I remember well, we though that it was not supported by ALTO - or just for the page. The attribute ROTATION at block level is however covered by ALTO for the content of block (<xsd:complexType name="BlockType"><xsd:attribute name="ROTATION" type="xsd:float" use="optional">) and should be part of the serialization.