Warn the user when loading a positivity fk with the dataset API

Created by: scarlehoff

Edit: changed the title to reflect what the actual bug is (tl;dr, user error)

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The positivity grids contain 20 points but instead the positivity predictions contain only 12 after the internal cuts happen. After the PC we believe this is a bug (or rather an oversight). It should be checked (whether it is a bug or it is intended) and then it should be fixed.

from validphys.api import API
from validphys.convolution import central_predictions
pdf = API.pdf(pdf="NNPDF40_nnlo_as_01180")
dat = API.dataset(dataset_input={"dataset": "POSF2U"}, theoryid=200, use_cuts="internal")
print(f"Positivity predictions {len(central_predictions(dat, pdf)}")