Vivarium

- Enable population manager and population view methods to accept strings
- Fix mypy errors: vivarium/framework/lifecycle.py
- Fix mypy errors: vivarium/framework/population/manager.py
- Fix mypy errors: vivarium/framework/population/population_view.p
- Fix mypy errors: vivarium/framework/plugins.py
- Fix mypy errors: vivarium/framework/results/stratification.py

- Fix mypy errors: vivarium/framework/randomness/index_map.py

- Pin Sphinx below 8.0

- Introduce static type checking with mypy
- Add new types for clock time and step-size

- Raise an error if a component attempts to access a non-existent population_view

- Update results docstrings
- Add a results concepts model doc
- Docstring cleanup throughout
- Fix up tutorial docs
- Strengthen doctests