Add CMSDY2D12 dataset

[cschwan]

This branch adds the CMSDY2D12 dataset from arXiv:1412.1115, with data at 10.17182/hepdata.69869. The runcards were copied from CMSDY2D11 dataset, with the following changes: beam energy increased from 3.5 TeV to 4 TeV, leading and subleading lepton pt increased to 20 GeV and 10 GeV, respectively.

Issues:

• the data available on hepdata.net seems not to include post-FSR data, only pre-FSR data (bad!)
• it is unclear which scale is used in NNPDF3.1. For the time being the same as in CMSDY2D11 is used
• In this dataset the first invariant-mass bin from 20 to 30 GeV is zero at LO because of the more stringent pt cuts on the leptons. At LO the smallest invariant mass in two times the leading lepton pt, which is 40 GeV, meaning that even the second is only filled for the last 5 GeV, that is 40-45 GeV

[cschwan]

For this specific dataset no dressed data is available and hence it is useless for our purposes.