Custom rdds: image stacks to series conversion

Conversion code is exposed in user API as context loadImagesStackAsSeries and context convertImagesStackToSeries. With shuffle=False, the default, it will use the "shuffle-free" conversion code added here; with shuffle=True, it will go via the images -> imageblocks -> stacks route.
Don't we need an extra flat map here, because at this point records are still grouped in a block? Something like the ``seriesrdd = blockedrdd.flatMap(blockToSeriesAdapter)`` we do within ``Images.toSeries()``?