Add BoundaryConditionDescriptorBase

src/scheme/AxisBoundaryConditionDescriptor.hpp

src/scheme/BoundaryConditionDescriptorBase.hpp

src/scheme/DirichletBoundaryConditionDescriptor.hpp

src/scheme/ExternalBoundaryConditionDescriptor.hpp

src/scheme/FixedBoundaryConditionDescriptor.hpp

src/scheme/FourierBoundaryConditionDescriptor.hpp

src/scheme/FreeBoundaryConditionDescriptor.hpp

src/scheme/IBoundaryConditionDescriptor.hpp

src/scheme/InflowBoundaryConditionDescriptor.hpp

src/scheme/NeumannBoundaryConditionDescriptor.hpp

src/scheme/OutflowBoundaryConditionDescriptor.hpp

src/scheme/SymmetryBoundaryConditionDescriptor.hpp