Add more summary statistics measures from {sjstats}

[IndrajeetPatil]

Maybe also some of the summary statistics from sjstats can be implemented in datawizard?

Originally posted by @strengejacke in #12 (comment)

[bwiernik]

My preference would be to implement a set of descriptive functions with general names (e.g., distribution_mean, distribution_sd, distribution_mad, distribution_cv) that consistently support a common set of useful arguments:

1. weights
2. trim
3. method (e.g., to compute maximum likelihood, unbiased, or robust versions of a stat).

and perhaps other arguments.

Small point: I really hate how "standard error" is commonly used to refer specifically to the SE of the mean, so if we added something like that, I would prefer it be something like distribution_sem() or distribution_se_mean().

[etiennebacher]

Where are we with this? Currently, the "Data properties" section contains these functions:

It would be useful to have a clear list of functions to add or import from other packages.