This reverts commit b82e6531.

for consistency.

This time only for sumfact, as we need direction dependency.

Which play well with the latest change to not assemble on the overlap.

We mistook a tag for an Id here...

It always aims for maximally horizontal vectorization
where possible.

The previous changes to directly pass the visitor
into the coefficient handler was messed up in one place
- only being triggered by the downstream maxwell example.

So far, it had to restore indexing information on the visitor
for the second branch. This can be avoided by index pushdown,
just as we do with the sum of two tensors.