Stochasticity in IDM and similar models

[tawheeler]

The intelligent driver model is traditionally free of stochasticity. The implementation in AutomotveDrivingModels includes a value for a Gaussian variance to add stochasticity. I propose that we decouple these concepts.

• We remove the variance from IDM and have it be deterministic
• We introduce a GaussianDriver that can include a submodel which produces the mean prediction. Thus one can include the IDM or any other driver model and introduce stochasticity on top of it.

Thoughts?

[tawheeler]

The typewise branch has a fix for this which implements my suggested approach.

mutable struct StochasticLaneFollowingDriver{D<:LaneFollowingDriver, P<:ContinuousUnivariateDistribution} <: LaneFollowingDriver
    submodel::D
    distr::P
end