Reorganize package content

[felixhekhorn]

The idea is to split the hard math away from the other administrative stuff (such as how to join operators etc.). This would be a small step towards #185 and eventually #189.

• This might also facilitate the implementation of LO polarised anomalous dimension #170 that would only act on this new side @adrianneschauss . Same would be true for FF (in the first stage) @t7phy .
• also N3LO ad variations #148 would mostly fall into the new side
• but instead Implement QED exact solution #135 should fall more towards the other, but in the current state actually does some further fixes on the math side

[alecandido]

This will also simplify #185, eventually.
In theory, the scope of that PR would become to move ekore and only it to Rust (in practice we might need several further adjustments because of numba)

[alecandido]

@felixhekhorn do you actually want a review now, or are you just attracting our attention on the topic?

[felixhekhorn]

no, not yet - I want to attempt the move into unpolarized/space_like/* first ... I'll mark this PR

[alecandido]

Maybe we should make a poll about how to sort external dimensions, currently:

1. anom-dim vs matching
2. polarization
3. time vs space
4. pto
5. flavor

I'm positive about keeping 4 and 5 in their current position, encoding them in modules and functions respectively. But I'm quite unopinionated on the first three.

[giacomomagni]

Test are passing, so these changes should be enough :)

[felixhekhorn]

• the most content of this PR is just a moving of files and the associated renames
• for the OME I had to alter a bit the structure to split the hard math away (which I called OME - I hope that is correct); @giacomomagni could you have a look please?
• we can decide to lift the bunch of raise NotImplementedError into the calling functions inside eko to save us some docstrings ...

For review: please @alecandido @niclaurenti @giacomomagni have a look now

• @enocera please feel free to have a look and leave a comment or review
• @adrianneschauss Polarized NNLO anomalous dimensions #195 should go smooth on top of this since I took bb6af22 from there
• @t7phy I think you can also build on top of this and start overwriting ekore.anomalous_dimensions.unpolarized.space_like.gamma_ns and the singlet counter part

The base branch was changed.