> the patch adds the `<iostream>` include to `elasticity_upscale.hpp`.

That's arguably worse.  There's nothing in `upscale_elasticity.hpp` that references symbols from `<iostream>`.

> `elasticity_upscale.hpp` includes `elasticity_upscale_impl.hpp` from within the `Opm::Elasticity` namespace

Instinctively that sounds like a strange choice. Any comments, @akva2?