Provide base() member for adapted views or locators (Trac 2229)

Moved from https://svn.boost.org/trac10/ticket/2229 description:

> I would like the result of `typename View::template add_deref::type` to include a member function `View& base()`, much like the iterator adapters currently do (and `std::reverse_iterator`).
> Imagine I have an rgb-to-hsv converted view, and then want to convert it back to rgb. All I should have to do is call `base()`.
>
> The actual use case: I have a deref function that exposes a reference to the element in a gray pixel.
> I also have a function that takes a view to regular types and creates an adapted view to gray pixels with that element type.
>
> I want to be ample to write them so that: `same_types(make_pixel(remove_pixel(T)), T)`