ENH:Expose PDF3DCalculator in Python bindings

[joeseaer]

Hi @sbillinge @dragonyanglong ,

This PR exposes the new libdiffpy PDF3DCalculator through diffpy.srreal.

It adds the Boost.Python binding for PDF3DCalculator, NumPy accessors for the
3D PDF and radial histogram outputs, binary grid export, delta pair filtering,
pickle support, and Python-side property wrappers for the 3D calculator
attributes.

This PR depends on the companion libdiffpy PR that adds PDF3DCalculator and
its supporting C++ helpers.

[sbillinge]

We are moving from boost to nanobind. This is on the nano branch which is what we should use as base branch.

Also, we will have to migrate the libdiffpy code also. Maybe merge the nano branch into 3DPDF branch?

[joeseaer]

Thanks Simon. I agree that this should move away from the Boost.Python wrapper path.

I checked the currently visible branches on diffpy/diffpy.srreal. I see migration-nano, but it still appears to use the existing Boost.Python binding code and I do not see nanobind usage there. Could you confirm the exact nano/nanobind branch or PR that I should use as the base?

Once the correct base is clear, I will prepare a clean PDF3DCalculator binding branch against that nanobind base, rather than continuing to extend the current Boost.Python wrapper in this PR. I will also keep this dependent on the companion libdiffpy PDF3DCalculator PR.