Remove using S for storage (use u)

[SouthEndMusic]

Fixes #968.

[Jingru923]

I see the new letter you are using is also used somewhere. For example, in equation section, equation (1) has also 'u'. In numerical considerations, equation (1), there is also 'u'.
For me, I think the doc is less confusing than before, it's improving. However, how do you think about the overlapping of 'u'? @SouthEndMusic

[SouthEndMusic]

@Jingru923 these all refer to the same $u$, which is the state vector of the model.

[Jingru923]

So the storages are states?

[SouthEndMusic]

So the storages are states?

Indeed, the system of differential equations which is solved by the Ribasim core is formulated in terms of the storages of the basins (and the integral term of the PID controller).