Example notebook: Gaussian process priors (#316)

* initial version of gaussian process priors example from st/examples (#234)

* include Literate in dependencies

* clean up GP prior script

* update

* Apply suggestions from code review

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* Apply suggestions from code review

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* add heading

* cell break markers for VSCode

* Apply suggestions from code review

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* clean up plots, add more kernels

* Update examples/gaussian-process-priors/script.jl

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* minor fixes

* two xrefs for kernel cut plots

* fix plotting

* more plotting changes

* plotting cleanup

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Author: st--