Add to_disv method to MF6 packages

[Huite]

Related to: #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:
Deltares/xugrid#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.

[Huite]

I've merged Deltares/xugrid#303 adding xu.UgridDataset.from_structured to xugrid (though not released yet).

(Testing here might be a good real test before it's released.)