Allow for flexible indexing of buses

[dmaldona]

The Jacobian matrix structure depends on the indexing of the buses and the way we order the variable vector (i.e. [v1 v2 a1 a2] or [v1 a1 v2 a2], etc.). Having control over this might help us both enhance the performance of the linear solver and decrease divergent scenarios.

Reviewers: @michel2323 , @frapac