ENH: Make XdmfAttribute, XdmfTopology, XdmfGeometry inherit from XdmfDataItem.  Instead of containing a data item, each is a dataitem of a specific type.  This makes setting data items more user friendly --- e.g. once you set the number of elements and set the topology type you don't have to set the number of elements from the dataitem again.  This still needs to be figured out for composite dataitems for things like sets.