851 preserve labels

@aashish24 @doutriaux1 Here's the new patch for the large-value contouring issue. I added a regression test for the isofill issue, and rebased it on your issue_851 branch.

The new baseline is in UV-CDAT/uvcdat-testdata#7.

Ref #851
are you sure about this @dlonie don't we use this option for some tests?