Unitful getters and setters

[luke-kiernan]

Add unitful getters and setters. Interface:

get_active_power(gen) # errors: must specify units
get_active_power(gen, {NU/SU/DU}) # natural units/system base/device base, with units attached 
get_active_power(gen, Float64) # system base, no units attached: internal usage.

5 + get_active_power(gen, NU) # errors: unitless combined with unitful
5 MW + get_active_power(gen, NU) # ok
0.5 SU + get_active_power(gen, NU) # errors: relative combined with natural

set_active_power!(gen, 0.5) # errors: must specify units
set_active_power!(gen, 0.5 {SU/DU}) # system base/device base
set_active_power!(gen, 50 MW) # natural units

Relies on IS #574. This will break stuff downstream left and right, yes, but here's why I think it's worth it:

1. Usability. It is very very easy to unknowingly mix units. The classic one: read in a time series from a CSV in natural units, then attach it to a system in SYSTEM_BASE.
2. Type stability. Our stateful unit system is inherently type unstable.
3. Maintenance. After the catch-all "not having adequate test cases," I've found more bugs due to units issues than anything else.

edit: at the moment, turns out 1-argument getters/setters assume system base. But I'd prefer to have them error: better that than get silently incorrect results. Also, I'd consider adding a 3-arg setter, with the units separate: set_active_power!(gen, 1, {DU/SU/NU}

edit: You can still mess up by adding or comparing device-base quantities from components with different base powers. Same goes for comparing SU quantities between different systems.

[luke-kiernan]

Here is perhaps the biggest argument for these changes:

This is profiling "add up active powers on all thermal gens in the 10k bus system:"

function sum_su_strip(gens)
    s = 0.0
    for g in gens
        s += IS.ustrip(get_active_power(g, IS.SystemBaseUnit())) # change this line
    end
    return s
end

[luke-kiernan]

Change get_active_power etc. to be non-unitful. Move unitful ones to a different name. Setters are fine as-is.

Does mean for get-then-set you need:
set_active_power!(gen, get_active_power_unitful(gen, SU))