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 don't clear then the close test fails because we need to do:
```
self._renderers = {}
```
Looking at the close test it seemed as if closing was clearing anyway. This might be another actually, try in current master:
```
x.plot(data)
x.close()
#works
x.open() # nothing happens
```