Strip out masked data before plotting vectors.

Looks like this is the best way to handle missing values in the
current VTK.

@doutriaux1 Can you test this and check that the results are as expected? Should be ready to merge then. I think this is the last of the vector plot issues.
@dlonie it looks good but I don't think we will have cellblanking, since this is a point only grid. right?