[wk-lib] iterate on Skeleton implementation

[jstriebel]

Add a new skeleton implementation which complies with the Design Doc. This might either be built on top of the current wknml-lib using loxun, or using the std-lib ElementTree, or lxml.

[jstriebel]

Further improvements after the initial implementation (#416) that we should look into:

• [x] cleanups in the NML import / export
• [x] should skeleton.Graph inherit from nx.Graph?
• [ ] discourage use of Graph/Group/Node inits (e.g. via warnings?) or allow users to initialize them by adding _skeleton (and maybe also the id) lazily when they are added to their parent
• [ ] add warning if exported graph is not a tree, add conversion methods
• [ ] check that _enforced_id is unused in user-code, e.g. using an error or warning that is handled in specific nml conversion scenarios