[FEATURE] Make the voltage source definition for the three-wire forms similar to four-wire ones

[hei06j]

Defining a three-wire fixed voltage source leads to degenerate set of constraints for the voltage source. This problem is already avoided in the four-wire formulation in

PowerModelsDistribution.jl/src/core/constraint_template_en.jl

Line 56 in 3014d49

if haskey(bus, "va") && !haskey(bus, "vm")

This could be ported to the three-wire forms.