Output subgrid levels jump after initialization

[Huite]

In this example, all subgrid elements are just offset by their bed elevation (so the water depth is constant across the area).

The basin water levels look fine:

But sampling over the subgrid levels, we get a jump at the start:

[Huite]

Likely related to: #903

[Huite]

Scratch that: the reason this happens is because the initial basin_level is at zero when the first interpolation occurs.

This is likely because of the order in which the callbacks are called, I guess?
At any rate, it means the first computed subgrid level is nonsense.

[Huite]

I think the best way to solve this is by setting the level right after creation of the Parameters struct, maybe?