Support make doctest similar to MDAnalysis/mdanalysis

[PriyaDiswar]

Is your feature request related to a problem?

Initially, I was going to work on the MDAnalysis/mdanalysis#3925 issue, and I was looking at psa.py. I noticed there was already a PR raised (MDAnalysis/mdanalysis#4378) to fix the doctest in that file. Later on another PR (MDAnalysis/mdanalysis#4403) was raised, which moved psa.py to this repository, and the previous PR was obsolete.

Describe the solution you'd like

From what I think, make doctest does not exist in this repository. Do you think we can support make doctest here? If yes, I would like to work on it.

Describe alternatives you've considered

Do nothing.

Additional context

None

[orbeckst]

In principle, doctests would be good.