Rewrite figure layout engine using new matplotlib features

[Cadair]

Provide a general description of the issue or problem.

Since this package was written many years ago two new relevant new things have been added to matplotlib, which could significantly simplfy and also enhance how we layout the figures.

Firstly is subfigures https://matplotlib.org/devdocs/gallery/subplots_axes_and_figures/subfigures.html if we replaced the main plotting axes with a sub figure then subclasses of the base class could make much more complex plots to be animated, such as animating a group of subplots.

Secondly the new subplot_mosaic functionality (https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.subplot_mosaic.html) has the potential to simplify how we layout the sliders and controls at the bottom of the figure.