Add Besag-York-Mollie notebook (#566)

* initial commit bym notebook

* internal references, bibliography, spell check, installs metadata, watermark, Chris' comments

* annotate dimensions, fix morris citation, added darkgrid to arviz, trying something with internal section references

* fix with-in document references

* removed temporary ICAR import

* added covariate analysis

* fix two typos

Author: daniel-saunders-phil