Fast wrappers for ImagePlus-Img-conversions

[maarzt]

This PR is required to improve the IJ1-IJ2 compatibility layer. It adds wrappers to translate between ImagePlus and ImgPlus. These new wrappers are faster than the existing ones. This is achieved by sharing buffers instead of copying and being lazily.

Add VirtualStackAdapter:

• It's similar to ImagePlusAdapter as it presents an IJ1 ImagePlus as an IJ2 PlanarImg, but it's way faster during initializing when used for an ImagePlus that contains a VirtualStacks.

Refactor ImageJVirtualStack:

• Code quality improvements.
• Fixing a bug when multiple ImageProcessors are used simultaneously.
• Adding ImgToVirtualStack#wrap method which simply takes an Img(Plus) and returns an ImagePlus.

Add PlanarImgToVirtualStack:

• Similar to ImageJVirtualStack but not as flexible. It shares buffers between the PlanarImg and ImagePlus instead of copying the planes.

Add ArrayImgToVirtualStack

• Similar to PlanarImgToVirtualStack, but works for 2d ArrayImg.

[maarzt]

@tpietzsch I finished integrating the changes you suggested in the review.

[tpietzsch]

@maarzt ImagePlusImg constructors still use now-deprecated (by you :-) PlanarImg constructors. If you find some time, it would be great if you could fix that...

[tpietzsch]

Releasing 2.0.0-beta-41 now ...