Allow for time-dependence of tilde kernel parameters in AffineMap and ConstantMap

[frankschae]

In order to not lose our Slack conversation @mschauer:

#34 restructures the SDE kernel.

Instead of AffineMap, we could define and overload TAffineMap
with TAffineMap(t, x, p) and B, β, σ̃ = linearparams(f, g, t) such that the tilde kernel can be defined as

kernel = SDEKernel(TAffineMap, ConstantMap, trange, (B, β, σ̃))

to support time-dependence.

[fmeulen]

that makes sense to me