refactor: make bus, bus2sub, and substations explicit inputs to build…

…_plant

prereise/gather/griddata/hifld/data_process/generators.py

@@ -133,8 +133,16 @@ def add_heat_rate_to_generators(generators, ampd_heat_rates, epa_needs):
     pass
 
 
-def build_plant():
-    """Main user-facing entry point."""
+def build_plant(bus, bus2sub, substations):
+    """Use source data on generating units from EIA/EPA/HIFLD, along with transmission
+    network data, to produce a plant data frame.
+
+    :param pandas.DataFrame bus: data frame of buses, to be used (after grouping with
+        ``bus2sub``) within :func:`map_generator_to_bus_by_sub`.
+    :param pandas.Series bus2sub: mapping of buses to substations.
+    :param pandas.DataFrame bus: data frame of substations.
+    :return: (*pandas.DataFrame*) -- data frame of generator data.
+    """
     generating_units = load.get_hifld_generating_units(
         const.blob_path["generating_units"]
     )
@@ -144,8 +152,6 @@ def build_plant():
     epa_needs = load.GET_EPA_NEEDS(const.blob_path["epa_needs"])
     epa_ampd = load.GET_EPA_AMPD(const.blob_path["epa_ampd"])
     eia_form_860 = load.GET_EPA_FORM_860(const.blob_path["eia_form860_2019"])
-    substations = NEED_THIS_FROM_TRANSMISSION_PROCESSING()  # noqa: F821
-    bus, bus2sub = NEED_THIS_FROM_TRANSMISSION_PROCESSING()  # noqa: F821
 
     # Filtering
     generating_units = generating_units.query("TYPE not in const.hifld_storage_types")