Port from pybind11 to nanobind

[wdeconinck]

This PR ports the pybind11 implementation to use nanobind instead.
Unit tests had already been improved with previous PR #32, to catch errors in porting, and are untouched here.

Especially noteworthy here are the new array-view interfaces which nanobind brings more naturally, with support for DLPACK etc.

[Copilot]

Pull request overview

Ports the C++/Python bindings from pybind11 to nanobind, updating build configuration and exposing nanobind-backed array/ndarray interfaces (incl. array API / DLPack-related hooks).

Changes:

• Switch CMake build from pybind11_add_module to nanobind_add_module and update dependency discovery.
• Replace pybind11 binding code with nanobind equivalents across all exposed Atlas types.
• Introduce nanobind ndarray-based views for Field (host/device access + NumPy/CuPy/DLPack entry points).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
src/atlas4py/CMakeLists.txt	Updates build to find/link nanobind and build the extension with nanobind_add_module.
src/atlas4py/_atlas4py.cpp	Rewrites the binding layer for nanobind and adds ndarray/array interop for fields.
pyproject.toml	Switches build-system requirement from pybind11 to nanobind.

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