`tree` method for groups

[Artur-man]

It seems the printing method for the hierarchy similar to zarr-python is missing in pizzarr ?
and if missing should this be a public method under ZarrGroup ?

import zarr
g1 = zarr.group()
g2 = g1.create_group('foo')
g3 = g1.create_group('bar')
g4 = g3.create_group('baz')
g5 = g3.create_group('quux')
d1 = g5.create_dataset('baz', shape=100, chunks=10)
g1.tree()
/
 ├── bar
 │   ├── baz
 │   └── quux
 │       └── baz (100,) float64
 └── foo
g1.tree(level=2)
/
 ├── bar
 │   ├── baz
 │   └── quux
 └── foo
g3.tree()
bar
 ├── baz
 └── quux
     └── baz (100,) float64