v0.10.0

Added

• A joint trait can jump at the split as well as along branches. traits.discrete(at_speciation=…)
  already worked inside simulate_joint, so a trait could drive speciation and change at each
  speciation — but nothing said so, nothing tested it, and Chapter 9 documented only the along-branch
  case, which made the combination look unavailable. It is now documented, with an example, and pinned
  by tests: a jump at a split is logged as on_speciation, a change along a branch as on_branch,
  so the two can be told apart when scoring a method. (#256)