Need DSBSparse diag_like constructor with densify_diag=True attribute

[vincent-maillou]

To run RGF without issue, we need to overcome the problem caused by the sparse storage of the diagonal (expected dense) blocks. This lead to numerical inaccuracy in the current implementation of DSBSparse.

• Unfortunatly we can't just allocate a n-D buffer array in RGF as we would lose the non-uniform block_sizes compatibility

• The list option is ugly, and we have a fancy data structure that should handle that

• [ ] Implement: diag_like(densify_diagonal:bool = True)

[vetschn]

@vincent-maillou I forgot if we already discussed this irl, do you think this is addressed sufficiently by the densify_blocks parameter in the from_sparray constructor? It gives correct results in practice but may be not the nicest solution...