More control over bins in plot_marginal()

[mayer79]

Currently, the binning of plot_marginal() is controlled by two parameters:

• n_bins: int = 10 and
• bin_method: str = "uniform".

Ideally, the user could also pass user-defined bins, or a string referring to a typical method to calculate the number of bins ("scott", "sturges", "fd"), just like the bins parameter of np.histogram():

We could rename n_bins to bins. If bin_method = "uniform", we call np.histogram(x, bins=bins). If bin_method = "quantile", bins would need to be an integer.

[lorentzenchr]

+1 for this feature. I'm happy to review.
Implementation should be fairly easy by using numpy.histogram_bin_edges inside bin_feature in binning.py.