Small improvement of function interpolate with single value

[natema]

https://github.com/worlddynamics/WorldDynamics.jl/blob/3fcc7400b9b393835e4b463d8ad3267b9021723c/src/functions.jl#L4

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?)