Add Datasets to netdiffuseR

[aoliveram]

I think it would be a great addition to include these datasets as built-in datasets available in netdiffuseR:

• 'New' study on product adoption among physicians:
  Iyengar, Van den Bulte, Valente, Opinion Leadership and Social Contagion in New Product Diffusion (2011). doi
• The study on smoking initiation in adolescent friendship networks:
  de la Haye, Shin, Vega Yon, Valente, Smoking Diffusion through Networks of Diverse, Urban American Adolescents over the High School Period (2019). doi