Mech vent preconditioning energy use & loads

- [x] Shared mechanical ventilation preheating/precooling energy use seems suspicious, need to investigate https://github.com/NREL/OpenStudio-HPXML/pull/763
- [ ] Reconsider whether the heating/cooling load should be reduced when preconditioning equipment is available or whether it should remain unchanged https://github.com/NREL/OpenStudio-HPXML/pull/753