====== Handler.calP(source,target) ======
Return the shortest  from source to target in a weighted graph G.
  - Parameters
  -   - source: node
  -   -   - starting node
  -   - target: node
  -   -   - Ending node
  -   - weight: string,optional(default='weight')
  - Return: path: list
  -   -   - List of nodes in a shortest path.
  - Raises: NoPath: if no path exists between source and target, return null.
  - Related function:
[[http://networkx.github.io/documentation/latest/reference/generated/networkx.algorithms.shortest_paths.weighted.dijkstra_path.html#networkx.algorithms.shortest_paths.weighted.dijkstra_path]]