System constructor does not properly initialize bus_numbers

[GabrielKS]

MWE:

The culprit is this line in the System inner constructor:

PowerSystems.jl/src/base.jl

Line 136 in 774695e

bus_numbers = Set{Int}()

, which initializes a System with an empty set of bus numbers that never gets added to except in handle_component_addition!.

Without objection, I propose to treat this as a bug: whenever we give a System to the user, get_bus_numbers(sys) should always correspond to get_number.(get_components(ACBus, sys)). The fix, changing that line to

bus_numbers = Set(get_number.(IS.get_components(ACBus, data)))

, does not seem to break anything. Without objection, I'll push that fix along with my implementation of #1231.