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

ENH: Allow XdmfGrids to insert XdmfAttribute elements.  Add this to the visitor logic to write to xml.

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 write() method to XdmfItem and implement in subclasses.  The write() method calls the visitor on the current object which performs some predetermined acivity (writing to heavy data, writing to light data, parallel output, etc.

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.

ENH: Added XdmfTopologyType to XdmfTopology to handle element types and associated const data like nodesPerElement and name.  This resembles "Replace subclass with fields" implementation in Refactoring and does a nice job encapsulating element type data in an enum like way avoiding unneccesary subclassing.