Add `to_disv` method to MF6 packages

Related to: https://github.com/Deltares/imod-python/pull/1245

@HendrikKok remarked that we currently don't have a DISV model with UZF package. For (integration) testing purposes, it would be quite useful to automatically transform a DIS model to DISV form and assert that e.g. head results are unchanged.

This is good reason to prioritize:
https://github.com/Deltares/xugrid/issues/8

Since for most packages, it should be as simple as calling as something:

new_dataset = xu.UgridDataset.from_structured(self.dataset)
return type(self)(**new_dataset)

For the "agnostic" packages nothing needs to be done I guess?
HFB stuff isn't really practical or nice; wells are probably doable, since it's just a matter of renumbering (but it needs the total number of layers and columns); might be most practical to add the method on a model so it can forward the dis package and convert everything.

Start by reviewing related pull request #1245 and xugrid issue #8, then inspect the MF6 package and model entry points to determine which packages need conversion. Done means DIS models can be transformed to DISV for integration testing, with head results asserted unchanged and special cases such as wells and HFB considered.