fillNodes should be an optional step

[ColinEberhardt]

The current public JS API adapts computeLayout to add a fillNodes step, which tramples over any existing layout properties, see #62

I think the public API should make this optional ...

computeLayout(..., false); // fillNodes turned off

Currently within d3fc we do not use the adapted version of the API.

[vjeux]

I don't like boolean arguments. Can you just make fillNodes exist only in the test environment and not expose it outside?

[ColinEberhardt]

Hmm ... you're right, thinking about it boolean args lack discoverability and don't self document.

I'll have a rethink.