Support VTK XML data files

[pletzer]

Can we support VTK XML data files? These might be more flexible and extensible than the legacy file formats.

1. What does it take to read/write VTK XML files?
   There is a vtkXMLReader class: http://www.vtk.org/doc/nightly/html/classvtkXMLReader.html
   There is also a vtkXMLWriter class: http://www.vtk.org/doc/nightly/html/classvtkXMLWriter.html
2. Does VTK XML support time?
   It looks like this is possible:
   http://www.visitusers.org/index.php?title=Time_and_Cycle_in_VTK_files
3. Does VTK XML support colors (COLOR SCALARS field in legacy)?
   It looks like VTK XML does support colors, at least to some degree.
   Here is an example:
   http://www.vtk.org/Wiki/VTK/Examples/Python/TriangleColoredPoints
4. Can one add user-defined metadata?
   It seems like this should be possible by adding information into a tag set, but I have yet to see any examples of how to do this. In fact, I have not been able to find any VTK XML data file samples.
5. What would a three.js VTK XML loader look like?
6. Galaxy framework data type support for VTK XML.
   I would estimate this to be between 2 - 5 days of work. One of the problems is that there seems to be very few sample datasets to inspect.

General comments"
I'm beginning to wonder if this should be set to a low priority. The general reason we began to explore this format was to allow us to get access to a dataset's field components when displaying a Galaxy tool. But it seems that most of the CSG community is still using legacy VTK datasets, so we would have to support them anyway. And unless we can get access to tool run-time parameters at the time we are setting metadata on the resulting dataset, we will be restricted to only having the number of components for each field (not what the components are) when using the legacy VTK data format.