v8+ to v7: porting remaining features

Right now v8 contains many features that I or my group would ever use. The list of features of OpenDSS is extensive, so that's expected. Still, porting remaining features will allow a proper comparison to decide whether we should completely drop v7 and use v8 exclusively.

If porting mostly everything is feasible, objective tests can be run to decide this. Ideally, porting the core PM code would allow using an existing, mature scheduler for distributed computing.

- [x] Explicity enumerate remaining differences (mostly very new code and PM-related)
- [x] Assess the feasibility of porting the code to v7