Scratch pyyaml for custom object export/import

sample

def minimal_representer(dumper, item):
    # type: (yaml.representer.Representer, spherical.item.Item) -> str
    """Minimal example of a representer that has features items will need."""

    tag = "!spherical.example"
    for i in spherical.utils.partially_sorted_partition(item):
        fields = [['id', str(item._id)],
                  ['title', item._title]]
    return dumper.represent_sequence(tag, fields)  # type: str


def minimal_constuctor(loader,       # type: yaml.constructor.Constructor
                       node          # type: yaml.nodes.Node
                       ):
    # type: (...) -> spherical.item.Item
    seq = loader.construct_sequence(node)
    # Problem - resulting seq lacks data: [[], []]
    return seq


yaml.add_representer(spherical.item.Item, minimal_representer)
yaml.add_representer(Workspace, minimal_representer)

yaml.add_constructor("!spherical.example", minimal_constuctor)