datatree

WIP implementation of a tree-like hierarchical data structure for xarray.

This aims to create the data structure discussed in xarray issue #4118, and therefore extend xarray's data model to be able to handle arbitrarily nested netCDF4 groups.

The approach used here is based on benbovy's DatasetNode example - the basic idea is that each tree node wraps a up to a single xarray.Dataset. The differences are that this effort:

• Uses a node structure inspired by anytree for the tree,
• Implements path-like getting and setting,
• Has functions for mapping user-supplied functions over every node in the tree,
• Automatically dispatches some of xarray.Dataset's API over every node in the tree (such as .isel),
• Has a bunch of tests,
• Has a printable representation that currently looks like this:

You can create a DataTree object in 3 ways:

1. Load from a netCDF file (or Zarr store) that has groups via open_datatree().
2. Using the init method of DataTree, which creates an individual node. You can then specify the nodes' relationships to one other, either by setting .parent and .chlldren attributes, or through __get/setitem__ access, e.g. dt['path/to/node'] = DataTree().
3. Create a tree from a dictionary of paths to datasets using DataTree.from_dict().