Update positivity and integrability metadata to conform with new pineappl version

[giacomomagni]

Update positivity and integrability polarized metadata to new pineappl standards.

[Radonirinaunimi]

Given that this requires rather more general changes, #81 should be included here (and is already). We may need to wait for NNPDF/pineappl#302 in case some of the crates are renamed (referring specifically to lumi->channels that would affect this line and this line).

[felixhekhorn]

I'm not sure you want to wait for pineappl v0.9, it might be good to have a working v0.8 version (for future reference). Then of course we have to have another PR for v0.9 (the moment it is available).

[giacomomagni]

I think we can merge this, as it corresponds to the branch used to generate the current theories-

[Radonirinaunimi]

Ok, agreed!