Documentation of keyword arguments

[tjstienstra]

Symmeplot higly depends on delegation by passing down keyword arguments down to plot objects -> symmeplot artists -> matplotlib artists. However, as pointed out in #32 this can be confusing. Therefore, it would be useful to improve the documentation.

I would propose a couple of improvements:

• Create a function to auto inherit docstrings' keyword arguments in the Scene.add_... methods. Make sure to end with a clear link to the matplotlib artist by which the other keyword arguments are handled.
• The type hinting of the submodules Scene classes such that the return type of methods like add_body is the plot object of the particular backend.