Decide how/if to have hooks for `.imag/.real` or general dtype specific attributes

[seberg]

The .imag and .real properties of ndarray return views into the original array. This means they cannot be described, e.g. by UFuncs. We could add an imag/real method/slot to the new DTypes which:

• Returns the offset into the existing dtype
• Returns a new dtype
  With that information, the ndarray can return the new view.

We could allow dtypes to tag on more special attributes (think quaternions, etc.). But it seems dangerous, at least for now. However, another thing would be to do this through some special new attribute on ndarray, such as ndarray.elementwise.<method>(*args, **kwargs) calling special dtype.arr_methods.<method>(array, dtype, *args, *kwargs) or similar.

[mattip]

Note that today np.ones(100, dtype='uint8').imag does actually create a zero-ed array