2D PlottableHistogram with kind='MEAN' (i.e. 2D profile plots) not supported yet

It seems writing 2D profile is not supported

```
import uproot
import hist

h = hist.Hist(
    hist.axis.IntCategory([10, 20, 30]),
    hist.axis.Regular(100, 0, 10),
    storage=hist.storage.Mean())

with uproot.recreate("test.root") as fout:
    fout['h'] = h
```

```
NotImplementedError: 2D PlottableHistogram with kind='MEAN' (i.e. 2D profile plots) not supported yet
```

Is it planned?