Serialize Vine

[wgurecky]

Proposed Enhancement:

• Method to serialize a vine copula to json
  • Should be possible using networkx (https://networkx.github.io/documentation/stable/reference/readwrite/json_graph.html) but might require additional machinery for node-to-edge linkages across trees.
  • need to store on each edge: copula name, copula rotation, kendall's tau, and optional; copula shape params. Also need to store edge-node linkages.
• Method to read serilized vine structure from json file back into a sample-able vine instance.