mf6 uzf: add a function to determine cell_id for the UZF package based on idomain

In GitLab by @Huite on Jul 18, 2022, 14:16

Based on the idomain, and a number of top layers, determine which cells should be used.

def uzf_cell_id(idomain, n_uzf_layers=1):
    ...
    return cell_id, landflag, ivertcon, top_or_not

Then vertical conductivity (vks) could be extracted via the cell ID.

Additionally, warnings or errors should be raised if the celltype is inappropriate (it should be phreatic), or when thicknesses in combination with surface depths are inappropriate.

I think imod.prepare is the most logical namespace for this.

Start in the imod.prepare namespace and review the existing UZF package preparation entry points. The work is complete when a function determines cell_id, landflag, ivertcon, and top_or_not from idomain and the layer count, and reports inappropriate cell types or thickness and surface-depth combinations.