Introduce plot method `mediation_model`#19

Draft

cedricbatailler wants to merge 4 commits intomainfrom

feature-plot-method

Commits on Jun 29, 2021

Commits on Jun 30, 2021

use `mediation_figure_options` in `plot.simple_mediation`
cedricbatailler
committed

Commits on Aug 6, 2021

merge `main`
cedricbatailler
committed