plot_mean_convergence subplots are numbered, not named

[gorj-tessella]

Each subplot in plot_mean_convergence is labeled with the feature index, not the feature name, as of version 5.6.3. This is because the features are internally specified by the num_vars variable, which is the feature index, without converting to the feature name with self._get_var_name_from_scalar(var).

https://github.com/AnotherSamWilson/miceforest/blob/d9359a89204e3b5f10cc02e7e621a22c213e5453/miceforest/ImputedData.py#L549-L552