ENH: Modify ItemProperties to use shared pointers like the rest of Xdmf.  Modify swig wrapping to correctly
handle both non const and const shared pointers.

ENH: Create a generic XdmfVisitor class and move writing operations to XdmfWriter.  Incorporate Loki library to perform asymmetric visitor operation (allows customizable behavior for eventual Xdmf / NetDMF library split).  Modify Loki
Visitor to work happily with boost::shared_ptr and modify swig wrapping to properly handle boost::shared_ptr inheritance.

ENH: Rename write() operation to accept() which is more consistent for the Visitor pattern.  Also, we can
do much more to our Xdmf graph structure than just write it using Visitor operations.

unnecessarily duplicated functionality.

ENH: Split visitor operation into a visit and a traverse operation so that logic of how to traverse the xdmf graph is kept with xdmf objects rather than the visitor.  The visitor calls this function rather than relying on its own logic.

	Add XdmfArray to store values --- modify XdmfDataItem to contain an XdmfArray.
	Modify Tests to use new XdmfArray.
	Modify XdmfVisitor to use new XdmfArray.
	Add Python Tests and move Cxx tests to their own directory.

ENH: Use libxml2 to do construction of xml file rather than using the crude stringstream way we were using previously.  XML does not own the data, it is used to make constructing XML files easier and libxml2 will be needed eventually anyway to parse xml files.

ENH: Allow for storage, retrieval, and output of datatypes other than int.  Incorporate into HDF5 output.

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.

ENH: Add XdmfDataItems to XdmfTopology and XdmfGeometry and write them out correctly in the XdmfVisitor.  Verified to work with simple Hexahedron file.