Naming Conventions: "_data", "data", "values", "_values"
A bunch of different classes have one or more of the attributes `data`, `_data`, `values`, `_values`, plus an assortment of `external_values`, `internal_values`, `formatting_values`, `get_values`.  These mean different things in different places.

Maintenance would be easier if the naming conventions were more uniform.  `Index` has all four of these attributes and I'm not sure there exists a nice backwards-compatible way to reconcile them with the naming in `Series`/`DataFrame`.  Any thoughts?  Does anything else think this matters?

(Motivating example: "Where are all the places in the code that touch a BlockManager.  Let's just grep for `\.data`...")

The lowest-hanging fruit for cleanup here is in the Accessor classes.  StringAccessor, SeriesPlotMethods, and FramePlotMethods all define `_data` to point back to their parent Series/Index, Series, and Frame, respectively.  I suggest that `_data` be replaced with just `_parent`.  The other two existing accessors `CategoricalAccessor` and `CombinedDatetimelikeProperties` use `categories` and `values` for these, respectively.  Ideally these would get standardized to `_parent` in the process.

Another option would be to change `NDFrame._data` to something like `NDFrame._mgr` so it there is little risk of name-overlap.  I expect this would meet more resistance than the accessor cleanup idea.