ColumnGroup.asDataFrame(), still gets rendered as ColumnGroup in notebook

[Jolanrensen]

Since notebooks render based on the runtime type, a column group cast to a dataframe still gets rendered as column group, which can look quite confusing.

Since we don't have a ColumnGroup<*>.toDataFrame() that does an actual conversion, there's no easy way to fix it.
Maybe we could add a function:

public fun <T> ColumnGroup<T>.toDataFrame(): DataFrame<T> = dataFrameOf(this.columns())

or change the rendering in notebooks to take into account the provided type first, then the runtime type (to catch Any? but actually DataFrame types)