Changes for 2.1.5

• Added VirtualizedPacked sample, which demonstrates a custom layout for large graphs of nested groups, defined in VirtualizedPackedLayout.ts. (Source is TS only)
• Fixed an issue where setting a node key reference in either a node or link via GraphLinksModel.setGroupKeyForNodeData, GraphLinksModel.setFromKeyForLinkData, or GraphLinksModel.setToKeyForLinkData could fail to resolve if the node did not yet exist. This issue did not exist during typical model loading.
• Fixed a regression since 2.1.1 with Diagram.clear failing to clear internal Parts lists (like Diagram.nodes).