Small improvement of function interpolate with single value

[natema]

WorldDynamics.jl/src/functions.jl

Line 4 in 3fcc740

interpolate(x, x₁, xₙ, y₁, yₙ) = y₁ + (x - x₁) * ((yₙ - y₁) / (xₙ - x₁))

I suggest to rename $x_n$ and $y_n$ as $x_2$ and $y_2$ to emphasize that it's just a line, and to typize the arguments (Number?)

[paulobruno]

I agree with $x_n \to x_2$ and $y_n \to y_2$.
But I would not sacrifice the readability of the function to explicit the type unless it's needed.