ENH: Allow passing multiple phases to potential references

[alihamdan]

A potential reference connected to a bus can be set on any (one) phase of the bus, in this case, the potential on that phase is fixed at 0V. Another type of potential reference exist for buses without neutral that set the sum of potentials of the bus to 0V.

We cannot currently use the second type with a bus that has a neutral. We cannot also choose what phases are considered for the second type. I suggest to change the PotentialRef constructor to allow the user to specify any phase configuration of the bus to be used as a reference of potentials.

In the docs, we should encourage the users to not pass the phases and rely on the default behavior as it always chooses the best configuration for any given scenario. In an "Advanced" section (to be created) in the docs, we can add a page that explains the advanced usages of the potential reference and show, using plots, the difference in the results.