Module step 

Step-function models: arctan, tanh, and erfc variants.

Structs

ArctanStep

Arctan step: A · (½ + (1/π) · arctan((x − x₀) / σ))

ErfcStep

Erfc step: (A / 2) · erfc((x − x₀) / (σ · √2))

TanhStep

Tanh step: (A / 2) · (1 + tanh((x − x₀) / σ))