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 NNPDF/pineko#53 (comment)

[alecandido]

A step forward has been done in #172, but there has been no check that the transition consistently happened in the code, and for sure there has been no transition in the docs.

[alecandido]

Q2grid has been dropped in 6b830a9

Verify the status of transition with:

rg q2 -i -l

Some occurrences of q2 are going to be kept for a while, since they're used in legacy compatibility.

[felixhekhorn]

even more: everything should be evolgrid by now