Add docstrings (and type annotations ?)

[mhugo]

The project is growing and the overall readability is not really enhancing ...

At the minimum add docstrings to document function usage and parameter types.

Maybe we should investigate the use of type annotations ? at least to highlight areas where the typing is a bit too much dynamic or not clear (It is already not very clear which functions take a regular dict from those which take PlotConfig)

[troopa81]

docstring seems a good idea. I'm unsure about annotations, I never used so maybe we can give it a try