When using `pdferr` in `plot_fancy` the pdf errors are used twice.

Created by: scarlehoff

If you use plot_fancy with use_pdferr: True in the runcard, the PDF errors will be added to the covmat. However, since the experimental errors are computed from the covmat and this is modified, the resulting error bands include the pdf errors as well. This is a problem since plot_fancy includes the PDF error already in the prediction.