COPOD explain_outlier: add option to add feature names to plot

Currently, the generated plot just shows Dimension 1, 2, ... etc. as the labels on the x-axis. This is not very informative. It would be nice if you could optionally supply a list of strings to the function, to add labels to the xticks of the plot.

This could look something like this:

def explain_outlier(self, ind, cutoffs=None, feature_names=None):  # pragma: no cover

    ...

    if feature_names is not None:
        plt.xticks(ticks=range(1, self.O.shape[1] + 1),labels=feature_names)
    else:
        plt.xticks(ticks=range(1, self.O.shape[1] + 1))

    ...