Design user experience of subset selection

[bbrowning]

How will we expose subset selection to users? Is it part of data mixing and Recipe yaml files? Is it a separate step outside of data mixing? Will we add any new top-level Python APIs to expose this? Or change existing ones, such as mix_datasets?

The purpose of this issue is to figure these things out and get feedback on that design.

[eshwarprasadS]

Thanks for this issue, @bbrowning

Some portion of this issue is also covered in #528