primod: Update mapping objects to update to iMOD Python's refactoring plans

[JoerivanEngelen]

I need to refactor iMOD Python's MetaSWAP objects, see Deltares/imod-python#1255. This has the following consequences for primod:

Part 1 of this refactoring consists of changing the well argument for the coupling objects from imod.mf6.wel.WellDisStructured to imod.mf6.mf6_wel_adapter.Mf6Wel. The latter is a direct data representation of the MODFLOW 6 WEL file. More can be read here Deltares/imod-python#728. This has the following consequences for iMOD Coupler:

• The imod.mf6.mf6_wel_adapter.Mf6Wel object needs to be provided to coupling objects as argument instead of the deprecated imod.mf6.wel.WellDisStructured.
• WellSvatMapping.self._create_well_id needs to be updated
• Test fixtures need to be updated.

Part 2 is a larger refactoring, where this Mf6Wel object will be an argument of the Sprinling.write and CouplerMapping.write methods, instead of the initialization. More can be read here Deltares/imod-python#1255. This will require the following updates to primod:

• In MetaMod.write, Mf6Wells need to be fetched from self.mf6_simulation and provided to self.metaswap.write.
• In MetaModDriverCoupling.derive_coupling, Mf6Wells need to be fetched from gwf_model
• Test fixtures need to be updated.
• More?

As I need to run primod against a dev version of iMOD Python, this also means I need to temporarily modify the pixi.toml, to use a install from git with pip, instead of a install from conda-forge.