Reworked DEMetropolis(Z) deep-dive notebooks (pymc-devs#509)

* 1st commit of reworked DEMetropolis(Z) drop tune fraction notebook.

* revised references.bib

* Commit with the second DEMetropolis (efficiency comparison) notebook updates and revisions

* Second push - first round of revisions - updated DEMetropolis notebooks

* run pre-commit

---------

Co-authored-by: Oriol (ZBook) <[email protected]>

Author: gbrunkhorst

examples/references.bib

@@ -581,6 +581,26 @@ @article{taylor2018forecasting
   publisher     = {Taylor \& Francis},
   url           = {https://peerj.com/preprints/3190/}
 }
+@article{terBraak2006markov,
+  title         = {A Markov Chain Monte Carlo version of the genetic algorithm Differential Evolution: easy Bayesian computing for real parameter spaces},
+  author        = {ter Braak, Cajo J.F.},
+  journal       = {Statistics and Computing},
+  number        = {16},
+  pages         = {239--249},
+  year          = {2006},
+  publisher     = {Springer},
+  url           = {https://link.springer.com/article/10.1007/s11222-006-8769-1}
+}
+@article{terBraak2008differential,
+  title         = {Differential Evolution Markov Chain with snooker updater and fewer chains},
+  author        = {ter Braak, Cajo J.F. and Vrugt, Jasper A.},
+  journal       = {Statistics and Computing},
+  number        = {18},
+  pages         = {435--446},
+  year          = {2008},
+  publisher     = {Springer},
+  url           = {https://link.springer.com/content/pdf/10.1007/s11222-008-9104-9.pdf?pdf=button}
+}
 @online{vandenbergSPSS,
   author        = {van den Berg, R. G},
   title         = {SPSS Moderation Regression Tutorial},