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.
No, we're okay here - I think the flat map you are looking for may be the one in _getSeriesBlocksFromStack? The series is certainly still grouped into a block, but that's up at the partition level - individual records are already in their final form coming out of _getSeriesBlocksFromStack.