Fix interpolation of cell fields with flying edges

Kenneth Morelandrequested to merge
kmorel/vtk-m:flying-edges-cell-fields into master

The flying edges algorithm (used when contouring uniform structured cell sets) was not interpolating cell fields correctly. There was an indexing issue where a shortcut in the stepping was not incrementing the cell index.

Fixes #799 (closed)

Backport: release

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