Allow non-root nodes to be used in partition & treemap

[jacobp100]

Fixes #91

[Misiu]

@jacobp100 what about other layouts? I think it's a good change, but it should work with other layouts too.
For example in tree. This would allow us to create this kind of layout:

[jacobp100]

Does d3 support anything close to that?

[Misiu]

Recently I had to build hierarchical layout for data without root (comes from external WS), I had to manually add root.
Having ability to specify non-root as source would make my life easier.
I'm waiting for ability to create layout extends, then having layout that I shown in my previous comment would be easy.

[jacobp100]

d3's hierarchy structure doesn't support not having a single root