Add "notes" field to saved model steps

[smmaurer]

This is a feature proposal to add a free-form notes field to saved model steps, alongside the name and tags. Originally suggested by @sablanchard.

This would be particularly useful in a GUI where we don't have a separate record of the model development process. But even in a Jupyter Notebook environment, it would be nice to have a way to embed free-form notes in the saved yaml files -- for when they're shared or just for general documentation.

The cost of adding this to the template spec is low, so i think we should go ahead and include it.

[sablanchard]

Great item to add! We have seen users use freeform string notes all the time to give context to versions of objects.