add docstrings

[PythonFZ]

Currently we do not lint for docstrings:

ZnFlow/pyproject.toml

Lines 45 to 50 in 378e8ff

	[tool.ruff]
	line-length = 90
	select = ["E", "F"] #, "D"] #, "N", "C", "ANN"]
	extend-ignore = [
	"D213", "D203"
	]

this should be changed to select = ["E", "F", "D", "N", "C", "ANN"] with as few as possible exclusions.
Doing so, will require adding docstrings and type hints to the code.