Defining steady state model not very intuitive.

[JoerivanEngelen]

After talks with @tessodk, we concluded that it currently is not so intuitive to define a steady state model MODFLOW6 in iMOD Python, as it:

• Still requires you to define a Storage package, where transient is set to false
• Still requires you to define a time discretization, with a start and end moment

I think we at least require some example to explain this, and possibly improve our API to ease this.

[Huite]

Note that we've made Storage required because the alternative is worse: if storage is omitted, MF6 automatically assumes steady-state. At least one user made the mistake where they thought they'd made a transient model, but it was running sequential steady-state. Including storage makes everything explicit.

MODFLOW 6 also has explicit time duration and time stamps (as floats), so it's not just xarray requirements for the start and end time.