5.0-dev2

transformer refactor changes (#121)

* transformer refactor changes

* remove base power from top level transformer

* name updates

* Migrate PSID serialized-system fixtures off the stateful units schema

Replace top-level units_settings (SystemUnitsSettings enum + base
value) with a flat base_power field, and rename the per-component
internal units_info key to base_value, matching the current PSY
serialization shape. Affects 9BusSystem.json, WECC_240_dynamic.json,
omib_sys.json, and threebus_sys.json.

* re-serialize latest

* typo