Cartesian vs Next example

[havogt]

Add an example illustrating using gt4py.cartesian and gt4py.next computations next to each other using gt4py.next storages.

Refactor GTFieldInterface and cleanup GTDimsInterface for next.

[egparedes]

I would wait to merge this after storages are merged so both versions can use similar allocation code.

[havogt]

I assume this Protocol should be used if users define their own buffers that should be treated as fields. __gt_domain__ is not enough, we need to specify how to get the buffer.

• Additionally: do want models of the concept to be also directly usable as field? or should this interface only be used to extract a buffer (that we could then wrap (possibly with host<->device copies) into a proper Field for embedded). Probably the latter makes sense, as the former is the Field interface.
• Note: forcing to add a member to a user object that returns an instance of a gt4py class, doesn't feel nice. Maybe we should at least allow -> DomainLike for __gt_domain__. Additionally, we would need Dimension to be any hashable object (similar to xarray) to be compatible in that sense.
• We should implement the same utility function for __gt_domain__ that we have for __gt_dims__: not access the property directly but via the function, then we can provide default implementation for known types or additionally provide a single dispatch point for extending the list of default implementations.

[egparedes]

It looks good to me. I only have a couple of questions.