Add ImmutableGraphIndex.isHierarchical

[marianotepper]

Adds a method to the interface ImmutableGraphIndex that returns true if the index is hierarchical and false otherwise.

Note that a graph can be hierarchical even if it has a single layer, i.e., getMaxLevel() == 0. For example, while building a new hierarchical graph, we may temporarily only have nodes at level 0 because of the random assignment of nodes to levels.

This change enables to make public the GraphIndexBuilder constructor that takes an existing graph as an input, as we can infer the addHierarchy member from the provided graph.

[github-actions]

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[marianotepper]

The alpha provided in the new public constructor is not really used by the builder. Something similar happens with neighborhoodOverflow. This may need a bit more refactoring to be sorted out properly. I'll label the constructor as Experimental for now until we have a proper solution.

[MarkWolters]

LGTM

[jshook]

LGTM