Move Cosserat physics to `elastic_spin`, following poroelastic.

[lsawade]

Task description

It seems to me that elastic_spin is more natural outside of the elastic folder at this point. With us working on 3D I believe it makes sense to have is_elastic_spin<...>/is_cosserat<...> and create either elastic_spin or cosserat folder. My preference would be elastic_spin due to the descriptive name instead of a "named" medium.

My main reason for this is that it would follow poroelastic setup.

I believe the initial though of having it in the same folder was that the elastic stress computation is the same. But it really isn't in our setup.

Acceptance criteria

• The tests for the new function bar() pass

[lsawade]

Effort is low mostly search and replace. the question is only whether to or not to.

[lsawade]

@maxlchien FYI

Linking: #1744