DEPR: Index.get_value ?
`Index.get_value` is a very strange (from user perspective), and completely undocumented method, so let's rename it to `_get_value` for internal usage and deprecate the public one?

Example usage to show its 'not-usefulness':

```
In [85]: idx = pd.Index([1, 2, 3])

In [87]: idx.get_value(np.array([0, 1, 2]), 2)
Out[87]: 1
```
(so you can index into another object based on the location of the key in the calling index)

