Fixes warning about initialization order in FullyImpliciteBlackOilSolver.
Merge pull request #81 from blattms/fix-fibs-initialization-order

Fixes warning about initialization order in FullyImpliciteBlackOilSolver...
Forward declare PhaseUsage as struct, not class.
Merge pull request #83 from atgeirr/fix-warning-struct-class

Forward declare PhaseUsage as struct, not class.