Simplify the mixing code to have only 2 dimensions instead of 3

At the moment the code always have one dimension of size 1, which is unnecessary, so we should simplify it.

assigned to @AlexBuccheri and @nicolastd

Note, the only other place calls to mixing are made is the Sternheimer code

added Refactoring label

@AlexBuccheri Just to let you know, I am finalizing this refactoring. If this is fine, I will assign you to for reviewing the corresponding merge request.

I didn't know you were working on this. Yes, makes sense to have me review

I didn't know you were working on this. This is why Gitlab is for, no? I wanted to let you know.

I am looking into redoing my own advanced version of real-space preconditioner, and I wanted to simplify the code first.

Ah, very nice. That should just plug in now, no?

Yes absolutely. I will most likely move your preconditioner code to its own module, in order to reduce the complexity in mix_t, and then the extra complexity of the preconditioning should be invisible from the mixing code itself.

I am not yet there, but I will let you know once this is done.

I was also thinking to split up the preconditioning into a separate module. Let if me know if you'd like me to do this as a separate MR that you can branch off of

mentioned in merge request !2379 (merged)

closed with merge request !2379 (merged)

mentioned in commit e010d102