Concentric graph support for smaller graph and graphs without all node types

[Garbash]

Concentric layout looks great but doesn't support two common use-cases: 1) Only a few nodes of each types exist, and displaying them in concentric looks weird: This takes a lot of unnecessary space

2) When Certain node type don't exist in the doc and this messes with view:

3) Distance between rings is shorter then max length of node title text in map and so nodes with longer text are hard/impossible to read. see issue #455

• When we fix this issue, we could switch default custom layout arrangement to ring, which would solve a problem with bootstrapping custom layout after a large node import.

[Garbash]

related to #492

[alon]

Requires a good definitions of what is readable or not. Requires a good test suite to verify readability for a large number of example graphs.