Add Domestic_Water_Heater hierarchy

[gtfierro]

Closes #777

Summary

• Adds Domestic_Water_Heater as a subclass of Water_Heater
• Adds 13 new classes covering the full domestic hot water equipment hierarchy:
  • Storage_Water_Heater → Gas_Storage_Water_Heater (Atmospheric_Gas_Water_Heater, Power_Vent_Gas_Water_Heater), Electric_Storage_Water_Heater, Heat_Pump_Water_Heater, Solar_Water_Heater
  • Tankless_Water_Heater → Gas_Tankless_Water_Heater, Electric_Tankless_Water_Heater
  • Indirect_Water_Heater
  • Semi_Instantaneous_Water_Heater
• Adds corresponding SKOS definitions in definitions.csv for all new tags

Design decisions

• Q1 (Indirect_Water_Heater + Boiler relationship): Added as a subclass only; the isHeatedBy relationship to Boiler is left for a future PR.
• Q2 (Condensing vs. non-condensing): Not added as subclasses for now — this mirrors how condensing variants exist under Boiler but keeping the initial hierarchy simple. Can be extended later.
• Q3 (Commercial vs. residential): Out of scope per the issue author's own recommendation.
• Q4 (Storage_Tank relationship): Storage_Water_Heater is kept as a distinct combined-appliance class rather than modeling it as a Water_Heater hasPart Storage_Tank.

Test plan

• python generate_brick.py completes successfully (51418 triples, no new errors)