Rename <crossSection>.ocpTrackID into ocpTrackRef

the attribute .ocpTrackID is defined as "xs:unsignedByte". This is impracticable and unintended. We should define it as tGenericRef and rename it into "ocpTrackRef".

c.f. forum entry http://www.railml.org/forum/ro/index.php?group=1&offset=0&thread=48&id=126 by Dirk