Improve luminosity plots

Created by: voisey

Closes https://github.com/NNPDF/nnpdf/issues/1012

After the change: https://vp.nnpdf.science/1Kr-J4nQQRGALHnaYW7BzQ==

Regarding implementing the uubar luminosity as @Zaharid suggested, I have a question for you @scarrazza since you wrote this code. I would naively implement uubar in analogue with how udbar and dubar are implemented (https://github.com/NNPDF/nnpdf/blob/master/validphys2/src/validphys/gridvalues.py#L136), i.e. by summing the situation when x1 corresponds to an up quark and x2 corresponds to an up antiquark and the inverse. But for qqbar (https://github.com/NNPDF/nnpdf/blob/master/validphys2/src/validphys/gridvalues.py#L125) this isn't done. Instead only the situation when x1 corresponds to a quark and x2 corresponds to an antiquark is considered. This is consistent with eq 4 of https://arxiv.org/pdf/1607.01831.pdf, where these things are defined, so I assume there's a reason for it, but I'm not sure what it is. Do you recall?