Vector field display

This is the PR to be able to visualize vector fields. This PR is based on PR#90.

I created a new interactor, largely based on what has been done for SH and tensors.
We spend quite some time with Olivier to be able to manage orientations correctly. I think it's ok now, but it would be nice if you could test with your own images, specially with images with none standard orientation.

Thanks,
Guillaume