Support multistep jobs generally

There are already issues for fragments (#131) and hybrid jobs (#132), this is to cover other types of multistep computations. The one I recently ran into was G3 in #656.

I think we've outlined some possible solutions in #131, and it seems each step should be a separate ccData object, and it remains to be seen if it should structured in a list, dictionary or some general object (I prefer this, since it looks like we'll go in this direction with attributes as well).