Calculate oil saturation from changes in water and gas saturation
First the change in oil saturation is calculated from changes in water
and oil saturation. Then oil saturation is updated based on this change
instead of just fixed to 1-sw-sg. With this change the oil saturation is
less sensitive towards numerical errors that may cause very small oil
saturations. Witch again may cause the simulator to think that the gas
phase is saturation with vaporized oil when it is not.
Add comments and change order for readability.
Merge pull request #200 from totto82/fix_updateState2

Update oil saturation from changes in water and gas saturations