TIEGCM Neutral-density fallback treats He as N₂ #1199

Neutral-density fallback treats He as N₂

When direct interpolation of QTY_DENSITY fails for all ensemble members, get_expected_upper_atm_density calculates density using O and O₂ mass mixing ratios, pressure, and temperature. Its mean molar mass formula treats 1 − O − O₂ entirely as N₂.

TIEGCM has included He, enabled by default, since 2.0. When He is present, this formula overestimates mean molar mass and therefore density at fixed pressure and temperature. This is separate from the pressure error below; the combined density bias need not be positive.