setup.py: install pxd and header files
This allows third party modules to subclass Kwant's cdef classes. https://cython.readthedocs.io/en/latest/src/userguide/sharing_declarations.html#search-paths-for-definition-files
Edited by Christoph Groth
This allows third party modules to subclass Kwant's cdef classes. https://cython.readthedocs.io/en/latest/src/userguide/sharing_declarations.html#search-paths-for-definition-files