Add boundary management in pugs' language.

src/language/modules/SchemeModule.cpp

src/main.cpp

src/scheme/BoundaryConditionDescriptor.hpp

src/scheme/IBoundaryConditionDescriptor.hpp

src/scheme/IBoundaryDescriptor.hpp

src/scheme/NamedBoundaryDescriptor.hpp

src/scheme/NumberedBoundaryDescriptor.hpp

src/scheme/SymmetryBoundaryConditionDescriptor.hpp