remove default view/layout sizes

[jrtom]

At the moment, JUNG provides a default size of 600x600 for layouts and views.

In my opinion, we should not be providing any defaults: if users want to create a visualization, they need to specify how big it should be. Ideally, this should be baked into how the visualization is created, but it may be necessary or appropriate to add Preconditions checks to validate that dimensions have been set.

This is partially a matter of practicality: 600x600 is a much smaller piece of screen real estate than it used to be when JUNG was created (because pixels are a lot smaller). If we were to continue to have a default, we might well want to change it to be a reasonable size. However, as long as we have a default, users may depend on that default being a specific size.

More fundamentally, though, I think that it's a mistake to specify a default when we can't make a meaningful informed choice on the user's behalf.

[tomnelson]

I have a PR ready for this but it has to merge after the lens PR #160 It also adds javadocs and makes the default spatial structure (or lack of) settable with a property.