Warn about phase labelling only above the water-oil contact

[GitPaean]

A single-zone compositional region (EQUIL item 10 = 1) warns when its composition does not vary across the gas-oil contact, as the flash then cannot label the phases on either side:

Equilibration region 1: the gas-oil contact at 2050 m lies inside a type-1 region, but the composition does not vary across the contact. Compositional variation is required for proper phase labeling.

The check looked at every cell of the region, including those below the water-oil contact, which hold water only. A region whose two contacts coincide has no liquid zone, yet it got the warning. The check now covers the hydrocarbon above the water-oil contact, so variation below that contact no longer counts either.

For a column from 2000 m to 2100 m with a constant composition and the water-oil contact at 2050 m:

Gas-oil contact	Hydrocarbon below the gas-oil contact	Before	After
2050 m, on the water-oil contact	none	warning	no warning
2030 m, above the water-oil contact	2030–2050 m	warning	warning

A gas-oil contact below the water-oil contact is rejected when EQUIL is read, so it does not reach this check. test_compequil covers both rows.