Add new variables to the SurfaceFlow struct

[hboisgon]

In wflow python, we use to have the water volumes in the kinemtic wave (m3) and the inwater flux (water volume entering the kinemtic wave in m3/s).

Would it be possible to add them again in Julia (vol and inw)? Would really benefit the coupling between wflow and Delwaq and also help with issue #90

By the way, I thought inwater is now qlat but the units of qlat in the end is m2 s-1 (in the strutc docs it is said m3 s-1).

Thanks!