ENH: DataFrame.name
Currently `Series` has a `name` member, which is useful for retaining the name of the sliced `index`/`column` when the `Series` is produced by slicing a `DataFrame`.

It would be useful for `DataFrame` similarly to have a `name` member, so that when a `DataFrame` is produced by slicing a `Panel`, the name of the sliced index is retained.

And while you're at it, may as well add `name` to `Panel` and even `Panel4D`...
