Function assemble 

fn assemble(
    shape: (usize, usize),
    blocks: &[Block],
    batch: usize,
    evaluate: impl Fn(&Block, usize) -> Result<[Option<Vec<Complex>>; 2]> + Sync,
) -> Result<DMatrix<Complex>>

Evaluate the blocks in parallel, in batches of at most batch entries or of one larger block, and scatter them in order. A missing second polarization shares the first; two missing polarizations skip the block. The evaluator also receives the worker budget of the current batch for nested lattice evaluations.