Reimplement Atlas Z0 8 TeV lowmass

[comane]

Possible bugs were found in the old implementation:

1. The overall multiplicative factor of the luminosity uncertainty given on hep-data (see https://www.hepdata.net/record/117854) is 1.8 %. In the old implementation 1.9 % was taken

-> I have incorporated this modification as it makes not a big difference

(this branch) https://vp.nnpdf.science/XpC1u8GqSyapV3e-ffvpwg==

(master) https://vp.nnpdf.science/7KddpIE6S9u2aBAfuTybkQ==

2. In the old implementation Lumi uncertainty is assumed to be MULT UNCORR. I have noticed that setting the Lumi uncertainty to CORR the chi2 grows by an order of magnitude. I am not sure I understand this. Perhaps in this case Lumi is supposed to be UNCORR?

[Radonirinaunimi]

Hi @comane, you need to run the regenerate-data bot to make the commondata test pass.

[comane]

A possible bug here is that the LUMI in the legacy implementation was treated as UNCORR.
It should however be CORR (ATLASLUMI12). If I set it to CORR, however, the chi2 changes by an order of magnitude (see report)
Am I missing something here or is it genuine?

https://vp.nnpdf.science/X59c09ozTEmm9vwP3vYTRg==