Drop Q2 everywhere

[alecandido]

We should update code and docs not to use $Q^2$ in place of $\mu_F^2$, in order to avoid ambiguity, and resulting confusion and bugs.

• fix docs
• fix code

Also $x$ would be better replaced by $z$ (but this is less crucial).

More details in #53 (comment)

[felixhekhorn]

be aware that there is still a slight ambiguity: from PineAPPL you get a mur2_grid and then when calling alpha_s you still need to do [as(xir*xir*mur2) for mur2 in mur2_grid], because PineAPPL really means the "default renormalization scale" (@cschwan correct me if I'm wrong). So if you want to go fully explicit one should call id default_mur2_grid or similar

[alecandido]

Ok, I meant to drop Q2 for whatever is needed, if fact scale, then muf2, if ren, then mur2, of course.

About the default_: I would assume is central by default. We can call the varied one mur2_varied, if ambiguous.
Since here the varied one is xir*xir*mur2, there is no need to rename anything.

[cschwan]

be aware that there is still a slight ambiguity: from PineAPPL you get a mur2_grid and then when calling alpha_s you still need to do [as(xir*xir*mur2) for mur2 in mur2_grid], because PineAPPL really means the "default renormalization scale" (@cschwan correct me if I'm wrong).

Correct.

So if you want to go fully explicit one should call id default_mur2_grid or similar

This difference is usually understood. If you really want to be explicit about it I'd suggest 'central'; the scale variations vary both scales around the central choice.

[alecandido]

If you really want to be explicit

I don't believe we need to be so explicit here. The grid we store is the non-varied one, and multiplying xir tells you once more.

Something we want to do, instead, is to perform consistently the replacement, and document it: we should write muf for fact and mur for ren, and specify for each function/method/attribute returning them what is returning, possibly remarking it is the central scale.