Change Density0 value and add comment

[xylar]

I think we should change the value of Density0 to 1035.0 and add a comment that it should only be used as the reference density that defines the pseudo-height:

Omega/components/omega/configs/Default.yml

Line 43 in ba2f369

Density0: 1026.0

We stated in the design document that 1035.0 was a good reference value:
https://docs.e3sm.org/Omega/Omega/design/OmegaV1GoverningEqns.html#pseudo-height