Preserve input subgrid_id in output

[visr]

What
The subgrid feature added in #674 takes subgrid_id [x,y,z], but does not preserve them, and writes them out as [1,2,3].

Why
We don't want to require allowing only 1:n consecutive subgrid IDs. So we need to preserve them.

How
We add the IDs to the struct Subgrid, and use these in subgrid_level_table as well. We should sort by subgrid ID as documented in https://deltares.github.io/Ribasim/core/usage.html#basin-subgrid

Additional context
Came up in #885 (comment)