This adds two new methods for working with EA-backed Series / Index.

- `.array -> Union[ExtensionArray, ndarray]`: the actual backing array
- `.to_numpy() -> ndarray`: A NumPy representation of the data

`.array` is always a reference to the actual data stored in the container.
Updating it inplace (not recommended) will be reflected in the Series (or
Index for that matter, so really not recommended).

`to_numpy()` may (or may not) require data copying / coercion.

Closes pandas-dev#19954

commit e4b21f6
Author: Tom Augspurger <tom.w.augspurger@gmail.com>
Date:   Mon Nov 12 16:09:58 2018 -0600

    TST: Change rops tests

commit e903550
Author: Tom Augspurger <tom.w.augspurger@gmail.com>
Date:   Mon Nov 12 09:31:38 2018 -0600

    Add note

    [ci skip]

    ***NO CI***

commit fa8934a
Author: Tom Augspurger <tom.w.augspurger@gmail.com>
Date:   Mon Nov 12 06:16:53 2018 -0600

    update errors

commit 505970e
Merge: a30bc02 3592a46
Author: Tom Augspurger <tom.w.augspurger@gmail.com>
Date:   Mon Nov 12 05:55:31 2018 -0600

    Merge remote-tracking branch 'upstream/master' into index-ndarray-data

commit a30bc02
Author: Tom Augspurger <tom.w.augspurger@gmail.com>
Date:   Sun Nov 11 15:14:46 2018 -0600

    remove assert

commit 1f23ebc
Author: Tom Augspurger <tom.w.augspurger@gmail.com>
Date:   Sun Nov 11 15:01:13 2018 -0600

    BUG: Ensure that Index._data is an ndarray

    BUG: Ensure that Index._data is an ndarray

    Split from pandas-dev#23623, where it was
    causing issues with infer_dtype.