Remove YAXArray Dataset type and replace by DimStack of YAXArrays?

[felixcremer]

On the way to removing our own types I am wondering whether we could remove the YAXArrays.Dataset type and replace it with a DimStack of YAXArrays? This would move us a step closer to removing our own Data types and currently the YAXArrays.Dataset indexing behaviour is a strange mix of the old YAXArrays indexing and the DimensionalData indexing and it is not clear – at least for me – how the indexing with a Dataset is supposed to be.