[scikit-image] Model PRs

Hi everyone I've started a new page on the wiki to track "Model PRs" (thanks, Juan, for the suggestion). These PRs are meant to illustrate how to perform certain common refactoring operations. I've added one example already, of how to deprecate modules that return XY formatted outputs to row-column format. If you have similar examples, please feel free to add them: https://github.com/scikit-image/scikit-image/wiki/Model-PRs Thanks Stéfan