Fixes for the scalable problem in CO and MDF

Fixed a number of problems with both solver and driver inclusion in a top workflow graph. Most of the problems were caused by:

1. Edges or nodes being incorrectly removed from the derivative graph at various stages while it was being processed.
2. Pseudo-assemblies didn't know not to clean out solver states.
3. If a solver's parameters are subvars, then we also need to turn their basevars into pseudo-assembly outputs.

Also, I deleted a duplicate test.