Fix breed methods

See https://github.com/NetLogo/NetLogo-Headless/issues/3. I have a number of commits here because I wanted to show the development of the refactor. They are all squashable upon request. The StructureChecker refactor (77494d1) is only partially complete. Depending on how we want to go with the refactor, I could:
 1) remove it entirely
 2) pull it over to another branch
 3) finish it on this branch before merging in
It was me who suggested the pattern match. See f5cae772daf4c6fc94c37ff60fc7d2d9b2d8160c. Still like the old version better?