fix: correct natural ventilation area unit conversion

[rcbevans]

Summary

• fix natural ventilation area conversion to use m^2 -> cm^2
• add regression tests for nat-vent unit behavior and guard conditions

Closes #219

[rcbevans]

This PR will close #219 when merged.

[jmaguire1]

This looks like a good catch! I need to take a little time tomorrow to confirm, but we do appreciate you catching this and putting in a PR!

For what it's worth: natural ventilation really only comes into play if you can basically get some free cooling from opening the windows, and in those cases it's a pretty large area that you can get airflow through, even without messing up the units. My guess is this has pretty small impact on the results and that's how it hasn't been caught until now, but I'll confirm that too.