Add support for additional column plotting

[fanshi118]

This PR adds an option to plot the sample means to be juxtaposed with the sample histograms and observed values in the datashader plot, as the sample means can be helpful when it comes to evaluating and comparing the results.

TODO's

• [x] add sample mean support in utils.py
• [x] update notebook example
• [ ] update docstrings in utils.py