[FEATURE] - Create VTK exporters

[JoerivanEngelen]

Part of the wishlist on our roadmap, we could do with some exporters to VTK to easily view things in 3D viewers like Paraview or PyVista. We already have a function to write structured grids to VTK for PyVista, but lack this for unstructured grids (DISV). This is low-hanging fruit.

Furthermore, we'd like to be able to export a complete simulation to VTK. Griddata will be exported to VTK files, and the model structure can be stored in a .pvd file for which both Paraview and PyVista have readers.

@FTW12345, mentioned he had some example code which can be our starting point. Furthermore I know Flopy also has some exporters we could take inspiration from.

[Huite]

I think I might've written this before, but: ideally the DISV (and in due time, DISU) pyvista mesh creation should live in xugrid. For the DIS meshes, there are two solutions: convert to equivalent DISV xugrid topology (but keep inactive cells, I guess), then convert to pyvista. Or do it directly. I don't expect much relevant difference in terms of performance, but it does matter in terms of the pyvista/VTK object we return (structured versus unstructured). Although I think the imod-python 3d stuff also produces an unstructured grid, maybe because this is more convenient in terms of visualization. For VTK export, however, converting to VTK structured seems like the better solution.

[Huite]

Some work here: Deltares/xugrid#419
This'll generate the 3D grids, but doesn't yet associate data with them.

Associating data is simple in principle, but converting to VTK files we'd probably want to stream the data instead? That is, write one time step at a time to minimize memory use.