Rect and mixed with ZIP loads

[luke-kiernan]

Fix the bugs noticed here: #431 Namely, update the functions that convert between PowerFlowData's fields and the residual vector such that they account for ZIP loads properly. Also add tests.

• Update functions that convert between data's fields and the residual vector such that they account for ZIP loads
• Add tests for the above
• Add guardrails for the cases those functions aren't equipped to handle (HVDC tails)
• Port the above over to the psy6 branch