add `boxBy` attribute

[hneutr]

put nodes in boxes based on an attribute

the way we would do this is:

• make "metanodes" of where each "metanode" is an attribute
• size that metanode by the number of nodes in it
• use d3 to simulate the forces between these nodes

then:

• for each metanode, run another d3 simulation for nodes in that metanode, and place them within that box

ideally, we'd also like to:

• be able to say "hierarchical" if it's a scalar we're boxing by, and also, to box ascendingly or descendingly in that case