Add type stub for cppcore

[anilbey]

Pull Request Template

Description

• Allows the argument and return types of the functions in cppcore to be understood by Python by providing the type stub file. Stub file is added to the package to allow downstream applications e.g. bluepyefe to recognise cppcore types.

• Adds mypy type checker to tox and github actions to check for type errors

• Fixes 2 small type errors

Checklist:

• Unit tests are added to cover the changes.
• The changes are mentioned in the documentation.
• CHANGELOG.md is updated (skip if the change is not important for the changelog).