This looks good to me. It would be perfect if you could also add a small test.

I'm working on fixing the signature.

Of course, you could drop the last commit and merge, but I believe to be valuable to avoid some overhead (whenever possible).

As you wish, since you are at it please add also the test that @cschwan asked for.

Thanks @alecandido

As a note for the future, we should add maybe a test that does the entire grid -> eko-convolution -> fktable and checks that the fktable can be read. Just to get a heads up in case any part of the interface changes.