Commondata has no len

Created by: Zaharid

I am running a comparefits report with the latest version of the code. This is failing here

https://github.com/NNPDF/nnpdf/blob/d87b26f14c480d9c7c5b7024223872587352f431/validphys2/src/validphys/fitdata.py#L326

with object of type 'CommonData' has no len().

@scarlehoff do you know what broke this (so I can run a working report quickly)?