don't lose voltage/phase values during topology invalidation

[jonenst]

Describe the current behavior

bus.setV(200)
System.out.println(bus.getV()); // OK

switch.setOpen(true);
switch.setOpen(false); // causes invalidation of buses (views)

System.out.println(getBusView().getBus(...).getV()); // NaN

Describe the expected behavior

like in powsybl-core, voltage/phase values should be kept as much as possible. this makes for better user interfaces where they can be displayed (maybe disabled) until new values are recomputed with the new toplogy

Describe the steps

No response

Environment

No response

Relevant Log Output

No response

Extra Information

No response