Improve against_apfel tests

[alecandido]

Cache

Compare with a cache of APFEL's results, if available

Split observables

Make a separate test for each observable: F2light, FLlight, F2charm, ...

Order by order

When introducing a new perturbative order the contribution of the new order should be compared with the correspondent from APFEL, instead of the full object.

Maybe the two things should be combined (if the correction is too irrelevant maybe it should not have to be too accurate)

Relative - absolute error

Probably no one of the two things make sense on its own, so maybe if the relative error is big enough also the absolute one should be checked against something else (what?), if it is too small maybe it's not really an error.

Candidates

• average over xs, Q2s
• some relevant standard point
• proposals?