Function to annotate each box with a small textbox, e.g. sample sizes

[trevismd]

Following up on that: It would be nice to have the option for additional annotations, since the framework to obtain the correct position is already in place. What I'm missing is a function to annotate each box with a small textbox, e.g. to add sample sizes to the boxes.

Originally posted by @jo-mueller in https://github.com/webermarcolivier/statannot/issues/51#issuecomment-647688954