[Improvement] Create a shorter array representation for ImageIO and Numpy arrays

[acsenrafilho]

What is the purpose here

As noticed in the general usage of data representation in the asltk code, the Numpy array is the highly used.
It would be nice if there is a more simple way to continue using the same function calls from Numpy, but directly using the ImageIO object.

For example:

1. Getting the array of data: img.get_as_numpy()[z,y,x]

Is there a shorter way to manipulate the Numpy representaion from ImageIO object?