add usage examples to artifact classes

[gregcaporaso]

Now that we can attach descriptions and usage examples to artifact classes and we have existing descriptions for most artifact classes in this plugin, we could take a pass through and add usage examples. We have a usage example for Phylogeny[Rooted] that could be used as a reference.

[lizgehret]

This could be a useful training exercise for gaining familiarity with our type system and usage examples. Adding this to the RSE1 board as a 'good first issue'.