add gibbskernel example to prior example script (#375)

* add gibbskernel example to prior example script

* examples/gaussian-process-priors/script.jl: gibbs example

make the GibbsKernel example more robust with thanks
to @devmotion and @theogf

* improve code formatting

Co-authored-by: Théo Galy-Fajou <theo.galyfajou@gmail.com>

Author: Cyberface

examples/gaussian-process-priors/script.jl

@@ -120,6 +120,7 @@ kernels = [
     LinearKernel(),
     compose(PeriodicKernel(), ScaleTransform(0.2)),
     NeuralNetworkKernel(),
+    GibbsKernel(; lengthscale=x -> sum(exp ∘ sin, x)),
 ]
 plot(
     [visualize(k) for k in kernels]...;