* Replace boost::python with pybind11

* Replace add_python_module with pybind11_add_module

* Fix clang error on type-dependent expression

#include statements to reflect this move.

--HG--
rename : tools/python/src/boost_python_utils.h => dlib/python/boost_python_utils.h
rename : tools/python/src/pyassert.h => dlib/python/pyassert.h
rename : tools/python/src/serialize_pickle.h => dlib/python/serialize_pickle.h

for other cross validation functions.