remove PartialPressureGas from SettingsObjectWrapper and reference qPrefPartialPressureGas
update files using SettingsObjectWrapper/PartialPressureGas to use qPrefPartialPressureGas
this activated qPrefPartialPressureGas and removed the similar class from
SettingsObjectWrapper.
Signed-off-by: Jan Iversen <jani@apache.org>