align BusTypes enum with PowerModels Int bus types

[claytonpbarrows]

We should make the BusTypes enum Int values align with the PowerModels bus_type fields. I.e. (BUS TYPE 2 bus type (1 = PQ, 2 = PV, 3 = ref, 4 = isolated))

see: https://github.com/NREL-SIIP/PowerModelsInterface.jl/pull/1#pullrequestreview-759040616