Mixed models: Include more info about nested and crossed random effects

[Pakillo]

See lme4 manual (http://lme4.r-forge.r-project.org/book/Ch2.pdf) and Bolker's article. Nested random effects (1|a/b) are actually equivalent to (1|a) + (1|b), as long as levels in b are clearly specified so they can't be confounded. See also https://m-clark.github.io/mixed-models-with-R/extensions.html#crossed-vs.nested