Add a /datasets/{dataset}/quantitationTypes/{quantitationType} endpoint

[arteymix]

It's currently using a query parameter, but a path parameter would be better. With single-cell data, there are additional endpoints we will be adding:

• /datasets/{dataset}/quantitationTypes/{quantitationType}/singleCellDimension, only single-cell
• /datasets/{dataset}/quantitationTypes/{quantitationType}/bioAssayDimension, either raw or processed
• /datasets/{dataset}/quantitationTypes/{quantitationType}/data, either raw or processed
• /datasets/{dataset}/quantitationTypes/{quantitationType}/singleCellData, only single-cell

I think it's good to keep /datasets/{dataset}/data/raw and /datasets/{dataset}/singleCellData for the preferred ones. The quantitationType parameter would simply be deprecated.

@oganm let me know what you think!