📄 add license headers

[burgholzer]

Description

This PR adds license headers to all relevant files in the repository.
To reduce the maintenance burden, a pre-commit check is set up that automatically checks for the presence of respective headers and adds them if missing.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[github-actions]

Cpp-Linter Report ⚠️

Some files did not pass the configured checks!

clang-tidy (v18.1.8) reports: 1 concern(s)

• include/mqt-core/python/pybind11.hpp:14:10: error: [clang-diagnostic-error]

  'pybind11/pybind11.h' file not found

     14 | #include <pybind11/pybind11.h> // IWYU pragma: export
        |          ^~~~~~~~~~~~~~~~~~~~~

Have any feedback or feature suggestions? Share it here.

[burgholzer]

Cpp-Linter Report ⚠️

Some files did not pass the configured checks!

clang-tidy (v18.1.8) reports: 1 concern(s)

• include/mqt-core/python/pybind11.hpp:14:10: error: [clang-diagnostic-error]

  'pybind11/pybind11.h' file not found

     14 | #include <pybind11/pybind11.h> // IWYU pragma: export
        |          ^~~~~~~~~~~~~~~~~~~~~

Have any feedback or feature suggestions? Share it here.

Ignoring this warning as it is unfixable at the moment since pybind11 is not installed in the cpp-linter environment.