Add a new key to a Step, named Artifacts. If present, it indicates that
the step has a "default" Kind and Container that can be easily retrieved
without explicitly specifying either when producing an artifact.
Add additional field MIMEType to Container.
This allows API consumers to know at runtime how to treat the data
within a container without any prior knowledge.
Adds missing API functions to PipelineC that will be needed by the
GraphQL API:
* Kinds enumeration
* Get Rank and parent of a Kind
* Enumerate and inspect Ranks
* Get a Step's parent
Included are some docstring fixes
Giacomo Vercesi