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Some style. Some fixes, especially for windows with the exports and template changes.

More so adapt this to work well as a subdirectory in a larger project. Don't force settings
that the parent project owns, allow parent project to turn on tests without xdmf's tests and
make the name of the library an option so that parent can have two xdmfs.

shared_dynamic_cast was deprecated and removed from newer versions of boost
as dynamic_pointer_cast provides the same functionality. Check at
configure time whether shared_dynamic_cast exists and if not, then
provide our own implementation that uses dynamic_pointer_cast.

ENH: Move XdmfCore into its own subdirectory.  Move XdmfCore specific tests into core subdirectory and modify
swig wrapping to create two separate libraries (Xdmf and XdmfCore).

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.