Add LHCB_WENU_8TEV data set

[enocera]

As the title says. WIP.

[cschwan]

• Replace set iappl 1 with set pineappl True, and make sure that you cherry-pick commit 2ecd40b
• See below

[cschwan]

* Replace `set iappl 1` with `set pineappl True`, and make sure that you cherry-pick commit [2ecd40b](https://github.com/scarrazza/applgridew/commit/2ecd40b8caa9cd68734677b5c4a5ea254fb18d1b)

The first is still missing, I think. You might also need commit f290569 to make the @MW@ substitution working.

[cschwan]

@enocera : Since I'm looking at the EW corrections for the datasets with large chi squares, I'm already running this set of processes.

[enocera]

@cschwan Very good, thanks. Although I doubt that large EW corrections are the culprit in each case where we see a large chi2.

[cschwan]

@cschwan Very good, thanks. Although I doubt that large EW corrections are the culprit in each case where we see a large chi2.

I doubt that as well, but I'm interested in how they change the chi square quantitatively.

[cschwan]

I tried to generate predictions for the two grids using the NLO QCD APPLgrids, but unfortunately my program crashes with

terminate called after throwing an instance of 'LHAPDF::RangeError'
  what():  Unphysical x given: 1

@enocera : Can you please try to get the predictions?

[enocera]

@cschwan OK, I'll have a look.

[cschwan]

@enocera : It's a small numerical problem, which sets the first grid point not at 1.0, but instead at 1.00000000000000022. In that case LHAPDF complains, of course. Manually setting the point to 1.0 fixes the problem.

[cschwan]

Check against NLO QCD, using NNPDF31_nlo_as_0118, showing differences in per mille (negative numbers mean that the APPLgrid result is larger):

• WP:
  30.93776 29.74231 30.30847 28.00807 27.80439 27.92609 26.00060 29.47064
• WM:
  31.81388 35.41998 33.11282 33.48928 36.19542 32.12345 37.10033 33.83617

Something is wrong here.

[cschwan]

For reference: this is the runcard for the APPLgrid: https://github.com/NNPDF/external/blob/master/MCFM-6.8/Bin/Input/LHCB_W_ENU_8TEV_WP.DAT

[cschwan]

Issue #72 could be a possible explanation: we don't cut on the neutrino!

[cschwan]

This (APPL)grid was generated without reweighting enabled, this could lead to loss of precision, see also NNPDF/pineappl#17 (comment).