driftFluxFoam: Rationalise phase-fraction handling and implement semi-implicit MULES

applications/solvers/multiphase/driftFluxFoam/UEqn.H

...ications/solvers/multiphase/driftFluxFoam/alphaControls.H

applications/solvers/multiphase/driftFluxFoam/alphaEqn.H

...tions/solvers/multiphase/driftFluxFoam/alphaEqnSubCycle.H

applications/solvers/multiphase/driftFluxFoam/calcVdj.H

...tions/solvers/multiphase/driftFluxFoam/correctViscosity.H

applications/solvers/multiphase/driftFluxFoam/createFields.H

...ications/solvers/multiphase/driftFluxFoam/driftFluxFoam.C

...ications/solvers/multiphase/driftFluxFoam/wallViscosity.H

applications/solvers/multiphase/driftFluxFoam/yieldStress.H

tutorials/multiphase/driftFluxFoam/ras/dahl/0/alpha1

...phase/driftFluxFoam/ras/dahl/constant/transportProperties

...ials/multiphase/driftFluxFoam/ras/dahl/system/controlDict

tutorials/multiphase/driftFluxFoam/ras/dahl/system/fvSchemes

...rials/multiphase/driftFluxFoam/ras/dahl/system/fvSolution

...rials/multiphase/driftFluxFoam/ras/mixerVessel2D/0/alpha1

...ftFluxFoam/ras/mixerVessel2D/constant/transportProperties

...iphase/driftFluxFoam/ras/mixerVessel2D/system/controlDict

...ltiphase/driftFluxFoam/ras/mixerVessel2D/system/fvSchemes

...tiphase/driftFluxFoam/ras/mixerVessel2D/system/fvSolution