update to vx/hierarchy

[trainorpj]

This is in reference to #136, which concerns the <Tree /> and <Cluster /> components. Links to components for convenience.

@hshoff addressed these concerns, but I wanted to list them here so we don't lose track of them. These apply to both components.

1. className is passed as a prop, but is never used in the component.

• Solution: That depends where className should be... I'm not sure, but probably in the <Group /> component?

2. size, nodeSize, and separation are all passed as props, but they are passed to their respective d3 layouts (d3.tree() and d3.cluster()). The defaults are not set in the components, but the d3 layouts sets defaults automatically.

• Solution: Set the defaults to be whatever we want, even if we hard-code them to be the d3 defaults... or just leave it be. I'm not sure what's best.

3. nodeComponent and linkComponent are passed as props, but they have no defaults. You can see that vx/network has default components for these.

• Solution: Just use the same defaults that are used in vx/network. They can just be put in vx/hierarchy/src.

I'm happy to make a PR for any of these! Just give me the word and I'll go ahead and do it. I can keep track of the docs when changes are made.

These aren't urgent problems, but they're worth keeping track of 😄