Extend sampling & serialized graph element features

[VJalili]

• Extend the list of features of sampled block nodes
• Update the sampling algorithm, making the following changes:
  • Implement both BFS & DFS expansions of node. Previous it was only DFS, BFS is more representative
  • Sampling decay is changed to be a percentage of node degree, where the algorithm samples n percentage of its neighbors
  • Set a MaxNeighborsPerNode option which allows capping how many neighbors per node should be included/expanded.
  • Add an option forceIncludeCoinbaseNode that if set, ensures the coinbase node is included irrespective of node degree thresholds.
  • Add a schema.json file alongside a batch of sampled communities that contains the schema of the features in the node and edge files in the sampled communities. This file simplifies parsing the feature vectors from the node and edge files.