remove the unused argument `df`

[baggepinnen]

df for APF is only used in smoothing, where it is used in the call

 logpdf(df, xb[m,t+1], f(xf[n,t],u[t],p,t), t)

to the custom method

@inline Distributions.logpdf(d::Distribution,x,xp,t) = logpdf(d,x-xp)

This PR thus has to

1. Not remove the argument from the docs completely
2. Require that a custom distribution or a custom logpdf function is supplied instead
3. Update the beetle tutorial to show how this is done