[BUG] reference to conductors that is not supported

[hei06j]

In functions relating to sequence voltage constraints, the command @assert(ref(pm, nw, :conductors)==3) throws error since :conductors is not supported. This bug is not seen because these functions are not used anywhere in the code and there is no unit tests for them.

A fix could be to replace it with @assert(length(ref(pm, nw, :conductor_ids))==3). However, since this assert restricts using this function to only three wire cases, and throws error is used for explicit neutral cases, perhaps a better fix would be @assert(length(ref(pm, nw, :conductor_ids))>=3).