TODO: add/replace figure with one command

[vfilimonov]

The reason: to avoid commenting add_figure() and adding replace_figure() on cell re-evaluation (e.g. when the figure passes via multiple iterations).

Alternatives:

• Make replace_figure() to add a new one if there're no figures on the slide
• Add replace=True argument to add_figure() that will search for the picture at the location where the new one is to be added, and if there's one - replace it. For example, matching could be done by comparing the overlapping areas of bbox and bounding box of pictures on the slide...