Positional constructors and Units #1822

Get positional constructors to accept units-bearing values. Right now, only the keyword argument ones do, which is a weird inconsistency.

There's also the question of whether units should be required, and if so, what that looks like. Taking ACBus as an example, the following args have implicit units:

  ACBus(
      number::Int,
      name::String,
      available::Bool,
      bustype::Union{Nothing, ACBusTypes.Value},
      angle::Union{Nothing, Float},                     # NU, radians
      magnitude::Union{Nothing, Float},                 # CU
      voltage_limits::Union{Nothing, MinMax},           # CU
      base_voltage::Union{Nothing, Float},              # NU, kV
      area::Union{Nothing, Area} = nothing,
      load_zone::Union{Nothing, LoadZone} = nothing;
  ) -> ACBus

Aside: it'd make things marginally easier on the user if our positional constructors had typed arguments. As-is, if you mangle the constructor call, the "closest candidates" are listed as ACBus(::Any, ::Any, ... ::Any), which isn't terribly helpful.