LeastSqFitter1d's constructor's order parameter is unsigned int. However it is stored as int and LeastSqFitter2d uses int in both places.
I suggest switching to int. The existing code always supplies a small positive integer, so it's safe and trivial to fix. However, it will require the same change to the pybind11 interface file.
I further suggest not changing this until after the pybind11 transition. Then change it and make sure the whole stack builds.