[vtkusers] serialize a vtk object to a xml string, not to save it to a file

[Xiaoping Chen]

Hello,

I am creating a client-server application. The server will be
Java-based, running in Linux and the client is C++-based, running in
Windows.

The server will map our internal data into the VTK objects and
serialized them into the xml strings and then send to the client.

I am able to do this in C++ but not able to do it in VTK Java wrapper.
 vtkXMLPolyDataWriter allows me to serialize the vtk object and save
it to a file but NOT allow me to turn it into a xml string.

Please help.

Eric