MinpvProcessor: opt-in thin-cell handling for edge-conformal grids

[hnil]

New defaulted parameter thin_cells_as_minpv (default false, behaviour unchanged). When enabled, active cells thinner than z_tolerance are treated like low-pore-volume cells throughout — removal, the column walk, NNC suppression and the gap check — and the merge branch runs for all removed cells, not only low-pv ones.

processEclipseFormat wires it to edge_conformal: edge-conformal grids select mergeMinPVCells and thin_cells_as_minpv so removed cells are merged geometrically and the grid stays topologically connected. No artificial faces via MINPV/PINCH NNCs (asserted), and pinch columns are treated as active gaps.

The requirement comes from the geomech/thermal stack: vertex-based discretizations need a conforming, connected grid, and NNCs across collapsed cells would create faces with no geometric counterpart.

Rebased onto master after #1051, which touched the same function. Two things were merged rather than taken wholesale:

• the off-the-bottom guard is now hoisted (from MinpvProcessor: guard merge branch against out-of-bounds zcorn access #1051), so the inner kk_iter == dims_[2] check this branch used to add is redundant and is gone;
• the NNC condition keeps MinpvProcessor: guard merge branch against out-of-bounds zcorn access #1051's collapsed-cell barrier and adds the thin-cell test in a form that reduces exactly to the old pv > minpvv comparison when the option is off, so the default path is unchanged rather than merely equivalent-looking.

All 82 opm-grid tests pass, including the 8 MinpvProcessor cases from #1051.