Improve code quality

[kralka]

Ideas:

• Set up automation of tests and code quality checks
• Add docstrings
• Improve formatting (yapf)
• Fix typing (mypy)
• Work in linter advice (pylint)
• Run spell checker

[kralka]

Fix typing idea: Use Literal type hint for split (bevare of Python3.7 compatibility) https://github.com/google/scaaml/pull/37#discussion_r779524015

[kralka]

General refactoring: Turn ['train', 'test', 'holdout'] into a constant in scaaml/io/dataset.py

[kralka]

General refactoring: Turn old formatting into f-strings.

[jmichelp]

FYI With PRs #63 and #64 being merged, I now enforced yapf to be green to merge any new PR.

[kralka]

It could be nice having dataclasses for parameters that we use everywhere together. For instance: https://github.com/google/scaaml/pull/75#discussion_r911128880