Use correct ratios for water isotopes

Currently we use arbitrary initial ratios for water isotopes: In WaterType.F90: SetupTracerInfo:

```Fortran
if (this%params%enable_isotopes) then
allocate(this%tracer_info(tracer_num)%info, source = water_info_isotope_type('HDO',0.9_r8))
this%is_isotope(tracer_num) = .true.
tracer_num = tracer_num + 1

allocate(this%tracer_info(tracer_num)%info, source = water_info_isotope_type('H218O',0.5_r8))
this%is_isotope(tracer_num) = .true.
tracer_num = tracer_num + 1
end if
```

We should change this to use typical actual ratios for these isotopes.