Add additional sum rules

Created by: Zaharid

We want to add more sum rules, it addition to those that involve physical constraints and add those to the tables in the report. This can be done by suitably adding definitions here:

https://github.com/NNPDF/nnpdf/blob/master/validphys2/src/validphys/sumrules.py

@stefanoforte please specify which additional sum rules precisely.