HyperSliceImgPlus Improvements

Hi,

as I ran into problems with the tests related to HyperSliceImgPlus I had a closer look at the implemenentation and tried to improve the HyperSliceImgPlus a bit.

* HyperSliceImgPlus now "really" acts as an ImgPlus with one fixed dimension, i.e. one less dimension. This means, that it also returns n-1 dimension when it's asked for the number of dimensions etc. Before it still had the dimensionality of the source image and also returned min/max/dimension values  for all the n-dimensions of the source, which was inconsistent behaviour and caused problems using some nested transforms.

* It uses Views.hyperslice which does basically the same

* It makes use of the ImgView class, such that we don't have to implement min/max etc.

* I fixed the copy method to also copy the image content (this is what you expect from an ImgPlus).

@tinevez are these changes fine with you? 

Best,

Christian