Document workflow classes

[innovate-invent]

I have been complaining for years for module and class level docs in Galaxy. This is my nudge in the right direction.

Relevant to @mvdbeek and @dannon

Please expand on the subworkflow functionality and how references are made between the different objects outside of the orm model.

[dannon]

Thanks for adding this, it's a good start and we should continue to expand on these docs as mentioned.