Increase the interchangeability of Process and Cut

Make cut names accessible via the 'label' attribute to allow for duck typing.

This change is not compatible with previous versions and breaks existing code.
When a cut is called with a dataframe, the selected events are returned instead
of an index array. To get the index array, use Cut.idx_array().

With the new behavior, cuts should return the events directly when called. This
change breaks existing code.