Optimize vtk

makes things a bit faster, still can be improved in another branch for template rendering and colorbar rendering.

@dlonie it is very similar to what you looked at before, should be able to review and merge this easily.

@aashish24 please take a look as well.

Don't forget to approve sister pull request: https://github.com/UV-CDAT/uvcdat-testdata/pull/8

ok will fix finalize for 2.1 (update this branch soon). Let's fix open for 2.2
We need to talk about context, brief discussion with @dlonie on wednesday it seems to be creating issues with OpenGL and multiprocessing in python (needed to optimze animations -other issue-)