Add standalone VTK file writing capability

[The9Cat]

VTK files are very useful for visualization. Many standalone visualization packages import them (e.g. ParaView, mayavi). The full VTK integration with CMake is HUGE to the point of unwieldy. It tends to pull in everything (including Java if one is using the Ubuntu package). And the only functionality EXP needs is file writing!

This suggests that we need a lightweight standalone VTK writer. The XML format is well described in the VTK documentation. This would not be very difficult for structured grids and other types if needed.