Move with_units here from PowerFlows.jl

[GabrielKS]

In a recent pull request to PowerFlows.jl, I've prototyped this function with_units that can be combined with the Julia do block to perform some action with a System under a particular units setting without permanently changing the units setting:

with_units(sys, PSY.UnitSystem.SYSTEM_BASE) do
  # things in this block will evaluate as if the System's unit system was SYSTEM_BASE no matter what it was before
end
# the System's unit system is back to normal

I think it should be moved to PSY and exported. Also

• A less heavy-handed approach to suppressing the logging should probably be taken
• Tests need to be written

Adding this as an issue here in case someone wants to take it on to learn about writing tests, etc. — otherwise I'll do it myself later this week.

[jd-lara]

We should add this to the power flow project