WIP: Resolve "SBGN - activity flow"

Closes #486

For reference: http://sbgn.github.io/sbgn/referencecards

TODO:

• Add NotOperator to model
• Add DelayOperator to model
• Add Activity element to model
• Add Perturbation element to model
• Draw Auxiliary Units according to SBGN standard
• Support the concept of an equivalence arc
• Support submaps
• Discuss the difference between AF Compartment and Complex
• Write a bazillion tests