Move "scalars_from" field to "input" and "input" to "bundles_from"

[tsalo]

Summary

This comes from a chat earlier today. If I'm recalling correctly, @mattcieslak proposed replacing "scalars_from" with "input" and moving the current "input" to "bundles_from".

I think both could be potentially lists, in order to produce nodes with all combinations of the scalars and bundles.

I'll try to add in examples to this issue soon.