destructors to be publicly accessible.  Modify tests to work with new wrapping.

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: Add pure virtual getItemTag method to XdmfItem working to return the tag (same as in XML terminology) for the specific item.

ENH: Add XdmfItemProperty to genericize properties attached to XdmfItems.  Add a pure virtual getItemProperties() method
to XdmfItem to force children to implement a way to retrieve properties.

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.

	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: 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 XdmfAttributeCenter to XdmfAttribute to specify how the attribute values are attached to the grid.

ENH:  Replace old way of creating static instances of XdmfTopologyType, XdmfGeometryType, and storing them by reference in their respective topologies.  These now work by providing a specific constructor method that returns specific instances of the objects populated with correct values.  This was changed because there are some type options in Xdmf where static instances don't make sense (e.g. XdmfTopologyType Polygon can have many different values for nodesPerElement) but we still need to restrict construction of these types with specific values.  Each parent item merely keeps a copy of a topology type.  I considered doing this with smart pointers, but I believe it to be unnecessarily complicated, especially when direct comparison of topology types is needed.  I am considering separating these types into separate classes.

Added XdmfAttribute implementation and began populating XdmfVisitor to write out XML.

ENH: Add XdmfGeometryType to XdmfGeometry in a similar implementation as XdmfTopologyType.  Add tests for XdmfGeometry.  Add XdmfItem, an abstract class that is able to be traversed by an XdmfVisitor to write out its information to XML and HDF5 or other heavy data formats.  Modify XdmfGeometry, XdmfGrid, XdmfTopology, and XdmfAttribute to inherit from XdmfItem.