It needs to:

• Allocate planar grids to layers
• Distribute planar conductances over layers
• River.from_imod5_data() accepts an allocation and a distribution setting
• Split infiltration factor into separate DRN and RIV package. See this snippet, not that this doesn't remove cells with zero conductance. The disv_converter also creates separate packages. Set drain elevation equal to river stage. (NB: River stage is defined with respect to model reference level, not river bottom elevation)
• The key of the original RIV package can get an appended, "-riv" and "-drn" suffix.
• The split into RIV and DRN package should be logged

Blocked by: #628