Support for dataflows

As discussed during the call, some of us (at least NYU) want to expose fine-grained pipeline details to TA3. This is not something that we expect every TA2 and TA3 systems to support, but should rather be an optional extension to the API, a common language for those systems that do support it.

This is a first version of the API for September, that does not yet include methods for editing a dataflow or changing parameters.