Save Taverna workflows as CWL

Many Taverna workflows can probably be represented in CWL - so this issue proposes to add support for serializing to CWL in Taverna Language.

I propose a staged approach:

a) Represent workflow structure only (A->B, B->C, B->D)
b) Save back CWL activities (from TAVERNA-877)
c) Save/Load other Taverna activities as TavernaRequirement
d) Save/Load other Taverna activities as DockerRequirement using TAVERNA-879

The semantics of CWL does not always allow all the iteration strategies of Taverna etc., so presumably only a subset of Taverna workflows could be saved correctly.