[BUG] More flexibility with figures

[jbschiratti]

In examples/examples/plot_minimal_pydeseq2_pipeline.py, we plot log-fold change versus mean expression using:

stat_res.plot_MA(s=20)

Currently, there is no easy way to adjust the xlim and ylim attributes of the figure displayed.

Similarly to what is done in Seaborn, what about passing an optional argument ax=None to plot_MA (and other plotting functions)? When ax is None, we let plot_MA create a figure and its axis. However, when Axes are passed to plot_MA, the method would use it for the scatter plot.