`T_density` defines `T0` in [Kelvin](https://github.com/JuliaGeodynamics/GeoParams.jl/blob/8b933994eeec6ac78a0a6e3799dd23511564e3e3/src/Density/Density.jl#L192) while in `PT_density` it is in [Celsius](https://github.com/JuliaGeodynamics/GeoParams.jl/blob/8b933994eeec6ac78a0a6e3799dd23511564e3e3/src/Density/Density.jl#L99). Perhaps we should have them all in the same units to be consistent.
