Cleanup of the plotables module

[pfebrer]

This fixes a bug: one could not pass **kwargs to the data generators when plotting through plot handlers.

Now:

# Passing kwargs to data from the plot handler
object.plot(data_kwargs={"arg2": 2})

# Is equivalent to
data = DataClass.new(obj, arg2=2)
plot = PlotClass(data)

in other words, data_kwargs is correctly expanded to pass the **kwargs for the creation of the data.

Most importantly, this PR rewrites the _plotables.py with plenty of documentation so that it is easier to modify in the future.

[zerothi]

Thanks @pfebrer, perhaps we should have some tests that reflects what is possible, and how they should work. I'll merge this, and work on my other tbtrans branch. Thanks!

[pfebrer]

Yes, definitely, and we should probably also discuss the behavior of **kwargs.

Right now, nodify workflows don't support **kwargs (nodes do support them). Therefore plots are not allowed to have **kwargs for now. So perhaps the best idea is to just pass the kwargs to the data function if they are declared. 🤷