Switch to nanobind

[fevangelista]

This pull request includes several significant changes to the wicked project, focusing on migrating from pybind11 to nanobind, updating the build system, and modifying the installation instructions. The most important changes are categorized into three themes: build system updates, migration to nanobind, and documentation updates.

Build System Updates:

• Updated CMakeLists.txt to require CMake version 3.15...3.26, introduced scikit-build for building, and added support for finding and using nanobind and Boost libraries.
• Removed the pybind11 submodule from .gitmodules and external/pybind11. [1] [2]
• Removed old build configurations from wicked/CMakeLists.txt and integrated them into the main CMakeLists.txt.

Migration to nanobind:

• Replaced pybind11 includes and namespaces with nanobind in various API files (api_wicked.cc, combinatorics_api.cc, equation_api.cc, expression_api.cc, index_api.cc, operator_api.cc, orbital_space_api.cc, rational_api.cc, sqoperator_api.cc, tensor_api.cc). [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Documentation Updates:

• Updated installation instructions in README.md to use pip install instead of setup.py.

These changes collectively enhance the project's build system, streamline the installation process, and modernize the codebase by migrating to nanobind.