Further diffusion models to include

[mamut86]

A potential list:

• NIU - Easingwood et al. 1983. A Nonuniform Influence Innovation Diffusion Model.
• Logistic model
• Weibull model - done
• Gamma model
• GMB - Bass, F.M et al. 1994. Why the Bass Model Fits without Decision Variables.
• Trapezoids and Triangles - Hu et al. 2018. Forecasting New Product Life Cycle Curves: Practical Approach and Empirical Analysis