Wildcard tagging

Currently the position and extends of a Tag need to match the referenced `DataArray`s. For certain cases it would make sense to have the Tag only specify a subset of the dimensions. For example a region of interest in an image stack, N 2-d images taken at various points in time `(x, y, t)`, would be specified in 2d (the image plane) and would apply to all the images in the stack. Wildcard, because the Tag basically would be `(x, y, *)`.