Fix CAPI before releasing PineAPPL 1.0

[cschwan]

While reviewing #337 I saw that pineappl_grid_new2 isn't completely general: we assume that there's always only one scale present. With that we also don't support general scales like ScaleFuncForm::QuadraticSum. This will require changing the parameters of pineappl_grid_new2 and therefore we must do that before releasing PineAPPL 1.0.

Furthermore, we should mark all functions deprecated that aren't needed any more.

[Radonirinaunimi]

Yes, I was planning to address this in #341. But if you would like to fix it soonish, that'd also be really great as that PR I'll be doing a bit slowly.