update to v4 and address comments

Author: OriolAbril

examples/gaussian_processes/GP-MeansAndCovs.ipynb

@@ -457,6 +457,15 @@ @article{nuijten2015default
   year          = {2015},
   publisher     = {Springer}
 }
+@unpublished{padonou2015polar,
+  title         = {{Polar Gaussian Processes for Predicting on Circular Domains}},
+  author        = {Padonou, Esp{\'e}ran and Roustant, O},
+  url           = {https://hal.archives-ouvertes.fr/hal-01119942},
+  note          = {working paper or preprint},
+  year          = {2015},
+  month         = Feb,
+  pdf           = {https://hal.archives-ouvertes.fr/hal-01119942v1/file/PolarGP\_CircularDomains.pdf}
+}
 @misc{quiroga2022bart,
   title         = {Bayesian additive regression trees for probabilistic programming},
   author        = {Quiroga, Miriana and Garay, Pablo G and Alonso, Juan M. and Loyola, Juan Martin and Martin, Osvaldo A},