Add units-aware getters/setters for power system components

[luke-kiernan]

Prototype of units-aware getters and setters. Core functionality works: some tests fail, but I believe it's due to inter-package friction (e.g. PSB's systems aren't quite compatible).

• Add conversion methods in components.jl for get_value/set_value
  • MW conversion: device_base * base_power → MW
  • Mvar conversion: device_base * base_power → Mvar
  • DU conversion: returns RelativeQuantity with device base
  • SU conversion: converts to system base per-unit
• Handle compound types (MinMax, UpDown) with recursive conversion
• Regenerate all struct files with new accessor signatures
• Update exports to include MW, Mvar, MVA, kV, OHMS, SIEMENS, DU, SU
• Update test dependencies and add explicit units API tests
• Dev InfrastructureSystems.jl dependency to local version

Example usage:
get_active_power(gen) # → 30.0 MW (default)
get_active_power(gen, DU) # → 0.6 DU (device base)
get_reactive_power(gen) # → 20.0 Mvar

[luke-kiernan]

Where things currently stand:

• set_value(comp, val::Quantity) calls the new Unitful conversion system
• set_value(comp, val::Float64) calls the multiplier system and uses the system's units setting
• get_value(comp, units) for fields marked as "needs_conversion" (e.g. active, reactive power) returns Unitful results; if units is omitted, you get natural units.

Main problems I've noticed:

1. Manual conversions: extra * base_power.
2. Implicitly using system base: since we're defaulting to natural units, need to change get_value to get_value(comp, "SU") in a couple spots. [We could change the default to system base, but then that gets weird when components aren't attached to a system.]
3. Constants: setting total_power = 0.0 then adding each power individually doesn't work any more, because you get unitless + unitful.

In PSY, the major places that use the old units system and will need overhaul are the ones we expected: parsing from PowerSystemTableData [planned for depreciation], time series, cost curves, and the public stateful API [set_units_base_system!, with_units_base].

edit: oh....an unfortunate side effect is that the getters now ignore the system's unit setting. That's not come up here in PSY--we don't manipulate units a whole lot here--but it will definitely cause issues in downstream packages.

edit^2: the above "unfortunate side effect" is fixable. Make the single-argument get_value(comp) return MWA, SU, or DU, depending on whether the system's unit setting is NATURAL_UNITS, SYSTEM_BASE, or DEVICE_BASE. However, that's immediate type instability.