V0.1.0

[lverweijen]

• UpTree is no longer exported by default, although it can still be imported.
  It has been removed from the documentation and is considered for deletion.
• Add AbstractTree.convert(obj) as a type-aware replacement for astree(obj).
  For instance, DownTree.convert(obj) can be used if obj.parent doesn't exist.
• Rename treeclasses.py to tree.py
• Rename conversions.py to adapters.py
• Don't special-case x in node.ancestors to use identity comparison.
• to_dot and to_mermaid now generate nodes in levelorder (breadth first), instead of preorder (depth-first)