Incorrect type returned from marl_eval.plotting_tools.plotting.probability_of_improvement

[liamclarkza]

The function marl_eval.plotting_tools.plotting.probability_of_improvement(...) is defined to return a Matplotlib Figure but actually returns an Axes.

We can either update the type definition or modify the function to return the handle to the Figure. I think the latter is a better option. Happy to put in a PR for either.

[liamclarkza]

Same issue also exists for marl_eval.plotting_tools.plotting.sample_efficiency_curves and marl_eval.plotting_tools.plotting.plot_single_task