update data sets used in examples

[rpruim]

Go through all examples and see whether they can be improved.

• give the examples a comprehensive look for value, consistency, etc.
• eliminate use of mosaicModel data sets
• eliminate iris and Galton
• eliminate other not-so-compelling choices
• eliminate examples that are not so useful
• add examples for features that could use an example but don't have one.

[rpruim]

I've replaced examples that had Galton or iris with uses of penguins.

I've left the Galton example in the blog post file from 2018 since that's they way it was when posted.

[rpruim]

I'm using {palmerpenguins} rather than {modeldata} now.