Extend `PyramidalDataset` to expose a particular item from its `List<SAC>`

[xulman]

The PyramidalDataset extends standard IJ2 Dataset mainly by featuring its public List< SourceAndConverter< T > > asSources(). This is great.

Next to asSources(), there's also accessible getting num* of channels, timepoints, and resolutions, so that a client code understands the content inside this PyramidalDataset. What is missing, is a mapping of a particular combination of timepoint and channel indices to an index in the List<SAC>. There could be a new method

ImgPlus getImgPlus(int timepointIdx, int channelIdx, int resolutionIdx) {
    //lists to the appropriate SAC and fetch the particular resolution from it
}

That way, the same memory should be reused!

[xulman]

It is probably better to move and implement the proposed method "one level upstream" into the Pyramidal5DImageData iface, because

• If Extend PyramidalDataset to get its underlying Pyramidal5DImageData #41 is implemented, one can "hop" one level up and get the ImgPlus from there.
• Implementations of the Pyramidal5DImageData iface are anyway those who understand the best how they have "serialized"/organized (e.g., Create List of SourceAndConverter without N5Viewer.buildSources #38) individual channels and timepoints into their own List<SAC> (see Pyramidal5DImageData#asSources())