CLN: final Panel cleanup

I think we can take the final step an remove all remaining references to Panel.

```
(pandas) bash-3.2$ grep -r Panel pandas --exclude '*.pickle' --exclude *.msgpack --include '*.py' |wc
     146     911   11617
```
note that the pickle  & msgpack files will stay.

TODO
- [ ] Have `_constructor_expanddim` raise a `Notimplementederror`