Interface cleanup

[ChrisBeaumont]

I am in favor of the following interface tweaks. None are super important, so feel free to veto any, @astrofrog

• [x] Remove Dendrogram.all_nodes
• [x] Move min_data_value, min_delta, min_npix into a Dendrogram.params dict (this is the style that scikit-learn uses)
• [x] Remove Structure.merge_level
• [x] redefine Structure.height to suggestion https://github.com/dendrograms/dendro-core/issues/25#issuecomment-19693352
• [x] implement Dendrogram.__getitem__ to fetch from node dict

[astrofrog]

Those changes all sound sensible to me!