[FEATURE] Update PF/SE assignments + solutions

[mgovers]

Describe the feature request

The existing power flow and state estimation assignments are outdated. For example, they contain a reference to unobservable state estimations for all grids, but the observability check for radial grids is already a necessary and sufficient check - only meshed grids potentially still give results even though they are not observable. This should be updated.

TODO: go through the power flow + state estimation solutions and update them. Then backport to the assignments.