Bug: Multi-slack not handled in DC but can happen if used for AC voltage init

[vmouradian]

Describe the current behavior

When using DC_VALUES or FULL_VOLTAGE voltage initializer with AC LF. The network can be created with multiple slack buses.
But then the DC LF used for phase init handles that in an incorrect way: all slack buses have no P balance equation and have phi = 0.
Gives false results and can even crash if a non-impedant branch connects two slack buses

Describe the expected behavior

Handle multi slack as in AC : one reference bus with phi = 0 and slack equally spread over all slack buses

Describe the steps

No response

Environment

No response

Relevant Log Output

No response

Extra Information

No response