fix: AbstractPlanarCommand
DefaultIOService passes context to ImgOpener

Instead of creating a new context each time ImgOpener is invoked, the
current context is reused.
Added xyPlane methods to DatasetView

Added xyPlane() and xyPlane(Interval) methods to DatasetView, and
default implementations in DefaultDatasetView.

These methods use the DatasetView's position to extract an XY plane
(assumed to be the first 2 dimensions) of the underlying Dataset/ImgPlus
(if no params) or the provided interval.