Redesign main interface

The main interface was designed prior to understanding the full extent to which it will be required to be used. As a result, lots of "hacks" have been done to get functionality. Rather than continue to hack, we should redesign the interface.

Related known issues:

• #111 (closed) (using n-parallel with multiple injections not supported)
• #116 (closed) Could at the same time allow for different dag-creation schemes