[waqtel] New variables dealing with oxygen saturation in TELEMAC result files

When coupling TELEMAC-2D or TELEMAC-3D with WAQTEL and using O2 or EUTRO, a few variables may be useful to be stored in the results file:

• concentration of oxygen saturation Cs,
• percentage of oxygen saturation defined as [O2]/Cs where [O2] is the concentration of oxygen.

Suggestions claimed by @anne.levasseur in #876

It will also enable to validate the new formula to compute Cs dependent on temperature and salinity (see #876).