Test XArray in combination with cattrs and dask (#62)

* Test some ways to unstructure xarray data

* Add dask and io to xarray optional dependencies

* Test how to read/write dask arrays

* Add test that shows that we need an unstructure_hook for DataTree

This is the simplest way to keep DataTree object the original.
Otherwise it would return a copy of some sort.

* Test that shows default unstructure behavior