use penguins data in greta examples

[njtierney]

The latest version of R will have the penguins data, so we should consider using the palmer penguins example where we can for some of the examples. We might need to consider which version of R we want to consider though.

[njtierney]

This probably won't really be relevant for a couple more years - once R 4.5.0 has been around for 2+ years we could consider this. So, shelving for the time being.