remove gridtype from dataset attributes

[dbrakenhoff]

Proposal by @rubencalje: Instead of checking gridtype we check for icell2d.

Explain somewhere in docs that this is the way to check whether you're using a vertex/structured grid. Maybe add a method get_grid_type(ds)?

[rubencalje]

In the future we will probably also support disu-grids. @martinvonk has some experience with these grids. This will introduce something like an inode-dimension, and remove the layer dimension. We can add the layer as an optional variable in the model dataset, for models that still consist of layers.

A dis-grid we call a 'structured' grid and a disv-grid we call a 'vertex' grid. How do we call a disu-grid? Or do we just check for an inode-dimension?

[OnnoEbbens]

Flopy uses 'structured' (DIS), 'vertex' (DISV) and 'unstructured' (DISU) so I would use the same terminology.