Support DataTree in xarray.concat

### What is your issue?

Something like this works, but is a little awkward -- especially the skipping over dimensions that are only defined on some node levels:
```python
def concat(*datasets, dim):
    if not all(dim in ds.dims for ds in datasets):
        return None
    return xarray.concat(*datasets, dim=dim)

xarray.map_over_datasets(concat, tree1, tree)
```