Allow any valid type as value for __setitem__

[TomNicholas]

Currently this works

dt["a"] = xr.DataArray(0)

but this fails

dt["a"] = 0

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Input In [9], in <cell line: 1>()
----> 1 dt["a"] = 0

File ~/Documents/Work/Code/datatree/datatree/datatree.py:704, in DataTree.__setitem__(self, key, value)
    700 elif isinstance(key, str):
    701     # TODO should possibly deal with hashables in general?
    702     # path-like: a name of a node/variable, or path to a node/variable
    703     path = NodePath(key)
--> 704     return self._set_item(path, value, new_nodes_along_path=True)
    705 else:
    706     raise ValueError("Invalid format for key")

File ~/Documents/Work/Code/datatree/datatree/treenode.py:444, in TreeNode._set_item(self, path, item, new_nodes_along_path, allow_overwrite)
    442         raise KeyError(f"Already a node object at path {path}")
    443 else:
--> 444     current_node._set(name, item)

File ~/Documents/Work/Code/datatree/datatree/datatree.py:684, in DataTree._set(self, key, val)
    682     self.update({key: val})
    683 else:
--> 684     raise TypeError(f"Type {type(val)} cannot be assigned to a DataTree")

TypeError: Type <class 'int'> cannot be assigned to a DataTree

The latter syntax is convenient and intuitive, so we should relax this to accept any type, and raise an error if the DataArray constructor can't parse it.

However it's not totally trivial to implement because it breaks some assumptions in the code that walks the tree.