copy subtrees without creating nodes for ancestors

[keewis]

Trying to copy subtrees leaves behind nodes for ancestors:

In [1]: import datatree
   ...: import xarray as xr
   ...: 
   ...: tree = datatree.DataTree.from_dict({"/level1/level2/level3": xr.Dataset()})
   ...: tree
Out[1]: 
DataTree('None', parent=None)
└── DataTree('level1')
    └── DataTree('level2')
        └── DataTree('level3')

In [2]: tree["/level1/level2"].copy()
Out[2]: 
DataTree('level2', parent=None)
└── DataTree('level1')
    └── DataTree('level2')
        └── DataTree('level3')

It seems this is caused by using absolute paths to copy over the descendants:

datatree/datatree/datatree.py

Line 670 in 49f7a2f

new_tree[node.path] = node._copy_node(deep=deep)

• Tests added
• Passes pre-commit run --all-files
• Changes are summarized in docs/source/whats-new.rst

[TomNicholas]

Thanks @keewis ! This was an oversight on my part - this change should have happened in #171

[keewis]

@TomNicholas, should we merge this, as well?