This entails:
- Adding a new data member of type Opm::Phase::PhaseEnum.
- Adding method getPreferredPhase().
- Adding extra constructor argument.
- Modifying constructor call in Schedule::addWell().
- Adding unit test for the new preferredPhase property.
- Modifying constructor call in all the well unit tests.
this is necessary because boost::gregorian::date does not have a
notion of "time during a day" which is required to specify time step
lengths less than a day...
