[waqtel] New formula to compute oxygen saturation concentration (Benson and Krause)

New formula to compute oxygen saturation concentration Cs, depending on water temperature and salinity (Benson and Krause). Implementation coming from @anne.levasseur New test case waq3d_eutro_streeter_phelps to validate the implementation.

One new choice for keyword FORMULA FOR COMPUTING CS (Benson and Krause). One new keyword WATER SALINITY in case there is no tracer SALINITY modelled with EUTRO or O2.