Handle parallel branch reductions for branches with phase shift

[m-bossart]

For certain types of components <: PSY.ACTransmission we might want to not allow double circuit reductions (e.g. phase shifting transformers). We can add a check in add_to_branch_maps! when the BranchesParallel objects are created

[m-bossart]

The current solution is to add a check and not add phase shifting transformers to parallel_branch_map:
https://github.com/NREL-Sienna/PowerNetworkMatrices.jl/blob/f29db913a65955b59b4ac8ff7e4163c0626e8d0f/src/Ybus.jl#L209-L214
This should be reexamined when developing modeling capabilities for phase shifting transformers.