Add a new notebook for SNMG benchmark runs (#4091)

This PR adds new notebook for SNMG benchmark runs. It
- [x]  simplifies bookkeeping and data presentation. 
- [x]  update each cugraph function wrappers to handle both SG and MG calls depending on a flag. The object is to have a common set of function wrappers for SG, SNMG and MNMG and put them in a common module to avoid duplication.

Authors:
  - Naim (https://github.com/naimnv)
  - Ralph Liu (https://github.com/nv-rliu)

Approvers:
  - Don Acosta (https://github.com/acostadon)
  - Brad Rees (https://github.com/BradReesWork)
  - Joseph Nke (https://github.com/jnke2016)

URL: #4091