update BEST pymc-example to PyMC v4 (#265)

* update BEST pymc-example to PyMC4 #52

- update to PyMC 4
- rename of greek variables spelling them
- add header
- update Autorhsip
- add License section

* update references.bib with refs of BEST notebook

* update BEST pymc-example to PyMC4 #52

- update to PyMC 4
- rename of greek variables spelling them
- add header
- update Autorhsip
- add License section
- use bib file for bibliography references

* Delete BEST.ipynb

* update references.bib with BEST example refs

* update BEST to PyMC4 following the reference style

* minor changes BEST example

* use of lowercase citekeys in references.bib

* use of lowercase citekeys in BEST case study

Author: asuagar