summarize to (optionally) use MEDIAN instead of MEAN as central summary

[tbhallett]

The summarize() function is hard-wired to use the MEAN as the central summary measure of the model results it is "summarising". 'mean' is also used as the name of the column giving the central summary measure; and only_mean is the kwarg used to get only the central summary measure.

... but sometimes we want the MEDIAN!

Simplest option would be to leave this function alone, and create a new one called summarize_with_median() that does similarly but uses median (for the stat, the name of the column and the kwarg). (Or, accomplish the same external behaviour through refactoring.) Or could create a kwarg stat: Literal["mean", "median"] that defaults to mean; and when median gives the title column of median, and we refactor the kwarg to be only_central instead of only_mean. (We could avoid wide-spread refactoring by also accepting an argument for only_mean, under certain circumstances.)