Add preferred phase to Well class.

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 needed because we want to use the preferred phase to set initial wellbore fluid composition for production wells.