ImgPlus should wrap any RandomAccessibleInterval

Currently, ImgPlus wraps an Img, but the same metadata it attaches would be useful to associate with any RandomAccessibleInterval, even if it is not also an IterableInterval. However, exactly how to proceed with this change requires some thought.

This change would enable the ImgSaver to work with non-Img objects such as those produced by Views, which would be very powerful.

Migrated-From: http://trac.imagej.net/ticket/1063

Start by reading the ImgPlus and RandomAccessibleInterval relationships, then trace how ImgSaver currently handles Img objects and how Views produces non-Img objects. The issue leaves the design open; done would mean ImgPlus can attach the same metadata to any RandomAccessibleInterval and ImgSaver can process those objects.

Written by the indexing model from the issue text.