Add convenience methods for creating images from arrays

[minnerbe]

The "learnathon" part of the ImgLib2/BDV hackathon in Brno showed that there is a need for an Img-creation API that caters to absolute beginners. The existing API for wrapping native arrays by means of the ArrayImgs util-class implicitly introduces the concepts of Img-backend and data-access, which seemed to keep people from exploring more awesome features.

I do think that wrapping native arrays is the best way to introduce people that are familiar with Matlab / Python / ... to ImgLib2, so I'm proposing this little wrapper API that can serve as a first exposure point for beginners:

ArrayImg<FloatType, ?> img = ArrayImgs.floats(array, dims) // previous
Img<FloatType> img = Imgs.fromArray(array, dims)           // new

Just to clarify again, this is meant as an educational API, so I don't see any need for deprecating the ArrayImgs API. This is simply inspired by the List-API that does a very good job of keeping implementational details hidden.

[minnerbe]

After discussing with @tpietzsch, it became clear that there is some potential to sustainably simplify static creation methods beyond this PR.
I created an according issue (#350) that supersedes this PR.