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

if you call close() and then open(), then you should create a new context. I would suggest we fix the open call. Let me know.