check for any exception.

* Replace boost::python with pybind11

* Replace add_python_module with pybind11_add_module

* Fix clang error on type-dependent expression

any object part labels so that the flipped dataset has the same average part
layout as the source dataset.  I added a --flip-basic that behaves like the old
--flip.  However, most people flipping a dataset with part annotations will
want to use --flip.