symmetrise SMEFT beta functions

[peterstangl]

This PR symmetrises the SMEFT beta functions in the redundant basis to fix issue #126.

[DavidMStraub]

Is there some document somewhere that can be quoted as reference? I'm not doubting its correctness (and I guess it can be checked for consistency to some extent by unit test), but a couple of years down the line nobody might remember how it was derived.

[peterstangl]

So what i did in this PR so far is quite simple: take all terms on the RHS, $T_{prst}$, and symmetrise them in $r\leftrightarrow t$: $T_{prst}\to \tfrac{1}{2}(T_{prst} + T_{ptsr})$. This reproduces the expressions on page 40 of arXiv:1312.2014, which are already symmetrised. The expressions involving Yukawas from page 15 in arXiv:1310.4838 are actually not symmetric in $r\leftrightarrow t$, but I also symmetrise those contributions.

[DavidMStraub]

Then perhaps just a detailed comment in the source code explaining that? I think it would be useful.

[peterstangl]

I think, in the long run, it might be good to completely regenerate this file from the .tex file of arXiv:1308.2627, arXiv:1310.4838, arXiv:1312.2014, arXiv:1405.0486, similar to what @jasonaebischerGIT and me did for the WET beta functions in #94. At the moment, the beta functions in this file are not simply those found in the papers listed above, but contain some simplifications/modifications that are completely undocumented and make a direct comparison with the original references rather cumbersome. As #126 shows, these modifications can also cause bugs that stay unnoticed for years.

After some archaeology, I found the original version of this file in https://github.com/DsixTools/python-smeftrunner/blob/6bf98dd5ca3d4048049140d5eaed1c3314c63cf5/beta.py, which seems to have been directly ported from the Mathematica file https://github.com/DsixTools/DsixTools/blob/14ee3da59e9f72de4354d54c39e7f2e9232b2e0d/DsixTools/SMEFTrunner/beta.m, which was used in the first Version of DsixTools.

For the moment (i.e. until we can regenerate and cross-check this file from the .tex of the original references), I will add several comments to the file, including the links to the original files in the DsixTools repository, and an explanation of the modifications done for the ee operator.