Add option to log exchanged river and drainage levels for Riba(Meta)Mod

[Huite]

Currently, it's quite a hap-hazardous task to figure out which levels have been communicated by Ribasim (via the subgrid levels) to MODFLOW.

1. Get the subgrid input from Ribasim (for meta_x, meta_y)
2. Get the subgrid output from Ribasim
3. Get the grid definition from MODFLOW
4. Re-do the work of primod in connecting subgrid ids to modflow boundaries: find the grid index
5. Index the subgrid levels via the grid index
6. Get the initial MODFLOW bottoms to remove below bottom levels; compute (max(bottom, subgrid_level)) for each subgrid id.
7. Hope/Pray you didn't get anything wrong and it's the same as the coupler internals.

Since the exchange flow logging is setup, it would hopefully be not too onerous to add some water level logging as well.