refactoring BCStrategy

apps/cpu/MultiphaseSymmetryTest/droplet.cpp

.../MultiphaseFlow/BoundaryConditions/MultiphaseBCStrategy.h

...aseFlow/BoundaryConditions/MultiphaseNoSlipBCStrategy.cpp

...phaseFlow/BoundaryConditions/MultiphaseNoSlipBCStrategy.h

...ryConditions/MultiphaseNonReflectingOutflowBCStrategy.cpp

...daryConditions/MultiphaseNonReflectingOutflowBCStrategy.h

...eFlow/BoundaryConditions/MultiphasePressureBCStrategy.cpp

...aseFlow/BoundaryConditions/MultiphasePressureBCStrategy.h

...phaseFlow/BoundaryConditions/MultiphaseSlipBCStrategy.cpp

...tiphaseFlow/BoundaryConditions/MultiphaseSlipBCStrategy.h

...ultiphaseFlow/BoundaryConditions/MultiphaseVelocityBC.cpp

...eFlow/BoundaryConditions/MultiphaseVelocityBCStrategy.cpp

...aseFlow/BoundaryConditions/MultiphaseVelocityBCStrategy.h

src/cpu/MultiphaseFlow/MultiphaseFlow.h

...low/Visitors/MultiphaseBoundaryConditionsBlockVisitor.cpp

...eFlow/Visitors/MultiphaseBoundaryConditionsBlockVisitor.h

...wtonianFluids/BoundaryConditions/NonNewtonianBCStrategy.h

...BoundaryConditions/RheologyBinghamModelNoSlipBCStrategy.h

...undaryConditions/RheologyBinghamModelVelocityBCStrategy.h

...Conditions/RheologyHerschelBulkleyModelNoSlipBCStrategy.h