Better defaults and more options for SBC plotting

[janfb]

the sbc_rank_plot function works fine but needs a refactoring and new features:

• the num_bins default seems to be off as it usually shows the CDFs with very large steps
• it would be nice to have SBC plots like in this paper, figure 4, e.g., eCDF as differences in a ellipse
• plus, general refactoring of the function according to software engineering best practices and UX.

[DhyeyJoshi39]

i've tried to add more functionality to this, please review it and give guidance if any mistake is there.

[janfb]

Thanks for the initiative here @DhyeyJoshi39.

I saw your PR and I will have a look at it later this week.