[python] Adjust to changed block_vector_unmanaged interface

This piece of code is nasty for several reasons:

• It's a dependency on the downstream module dune-istl.
• It depends on implementation details.
• For the specific details it is very explicitly noted that they should not be used outside of dune-istl.

But maybe this is unavoidable for technical reasons here.

mentioned in merge request dune-istl!560 (merged)

This is needed for dune-istl!560 (merged).

This is similar to the type-traits DimRange removed recently with dependencies to dune-typetree. Probably this is here because extracted from one big repository long time ago. I guess this template specialization can be moved to dune-istl.

That is my guess as well - this file is by Martin Nolte from 2017 - so the specializations should probably be moved from here to dune-istl. Perhaps this MR can just be merged and then a new MR setup to move things at a later point in time?

But this change is unrelated to dune-istl!560 (merged). Hence it should rather be done separately.

Why is this unrelated? Currently, the base_array_unmanaged expects as second template parameter something that has a ::size_type. So, merging this separately will probably break some code in the dune-istl python bindings, right? We should merge them along-side with dune-istl!560 (merged) I guess.

I like both changes, so I would be fine to merge both.

Why is this unrelated?

This is a misunderstanding. I meant that the move of the bindings code is unrelated to dune-istl!560 (merged) - not the present MR.

The IsBlockVector type-trait is not used anywhere (at least in my local module collection). There is a similar-named type-trait in dune-fem, but it seems unrelated. Maybe we should merge this change if it is blocking dune-istl!560 (merged), but eventually we should just remove (or at least deprecate) this utility.

Ok, this last comment was not quiet correct. The IsBlockVector is used in IsOneTensor that is used in registerOneTensorInterface (to protect from instantiation with non-block-vector types) that is called in dune/python/istl/bvecto.hh to register a block vector to python. Thus, moving the type-trait to dune-istl is still fine.

Also notice, that without this patch the BCRSMatrix python test in dune-istl fails. Otherwise I would not have noticed this.

Does this patch work with the current dune-istl? If yes, we should merge it and discuss a cleaner separation of the modules on the python side later.

It does not. The present code goes hand in hand with dune-istl!560 (merged). But the modified code is not tested in dune-common itself. Thus we should merge the present MR first. This will not break dune-common but downstream dune-istl until dune-istl!560 (merged) is merged.

merged

mentioned in commit 43290c5e