Output class

Here's the first version of a class to handle output. Nominally it supports .vtk and .bov although the .bov code is not currently functional. The .vtk code works in parallel, but yields output that isn't automatically stitched together in VisIt. This is good enough for now, but eventually we will want a more convenient output format.

I also added Doxygen comments for the new class because I think it'd be a good idea to use Doxygen for the code documentation (or perhaps Doxygen+Sphinx).