Update SG notebook (#4169)

Update SG notebook to handle a handle a couple of cases where the current notebook would fail for unweighted graph and  if the graph doesn't  have node id 0.

**Changes for SG notebook:**
- For graphs without weights, run bfs instead of sssp.
- For the input graph doesn't have node with id 0, peek an existing node id as seed for BFS and SSP
- simplify bookkeeping for run statistics

**Changes for MG notebook:**
-  Call SSSP or BFS based on weighted attribute

Authors:
  - Naim (https://github.com/naimnv)

Approvers:
  - Brad Rees (https://github.com/BradReesWork)
  - Don Acosta (https://github.com/acostadon)

URL: #4169