add data library example for jags

[vandenman]

Fixes https://github.com/jasp-stats/jasp-test-release/issues/2997

The example jasp file could be a lot better/ more informative, but for that we'd need multiple datasets, or a way to export multiple columns from within JAGS. Since those columns need not be the same length as the existing data, this is to be too tricky for the upcoming release.