Python type hints

This release adds type hints to the Python version of Astronomy Engine.

Type hints provide better documentation to developers about all the function parameter types and return values. They also enable static type-checking of the Python code using the mypy tool. The automated test suite now requires passing mypy --strict when each change is pushed to GitHub. This extra step will help maintain a high standard of code quality for future development. The intention is to prevent bugs before they happen, both inside Astronomy Engine and in code that uses it.

Many thanks to @ris-tlp for volunteering to help in this effort!