Added safe weighting function with optional unsafe version

[JonasIsensee]

Docs still need to be changed.

Question: Which version should be the default?

[JonasIsensee]

@Datseris
What's the point of this parametrization? F should always be a function.

struct LinearLocalModel{F} <: AbstractLocalModel
    n::Int #n=0,1,2,3
    ω::F
    f::F
end