Add HSGP reference and example nb (#647)

* initial commit

* update w chris' comments

* Merge BIll's an my changes

* Rerun up to example 3

* Improve some plots

* Finish improving Kronecker

* Fix bug in L computation

* Revamp example 1

* Split HSGP example into 2

* Improve references

* Remove autoreload

* Add Juan's comments

* Update with latest changes in PR

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Co-authored-by: AlexAndorra <andorra.alexandre@gmail.com>

Author: bwengals