====== Handler.writejson(cid1,cid2,pathList) ======
  - Returns a json file shown on the website. It is list of shortest path from source company to target company.
  - Parameters:
  -   - cid1: company id, starting node
  -   - cid2: company id, ending node
  -   - pathList: list of shortest path
  - Returns: four list of path
  - Raises: NoPath: if no path exists between source and target, return null.
  - Notes: give an example.