Move to clearer src/{python,cpp} structure.

setup.py

src/cpp/foreign_array.hpp

src/cpp/foreign_array_wrap.hpp

src/cpp/predicates.cpp

src/cpp/tetgen.cpp

src/cpp/tetgen.h

src/cpp/triangle.c

src/cpp/triangle.h

src/cpp/wrap_tetgen.cpp

src/cpp/wrap_triangle.cpp

src/python/__init__.py

src/python/common.py

src/python/tet.py

src/python/triangle.py