Theory covmat for higher twist

Created by: RoyStegeman

We want to add a covmat to account for higher twist effects. Therefore the covmat should be constructed similar to a 3pt prescirption where \Delta^\pm = \pm \sigma_\mathrm{obs}\frac{c}{Q^2(1-x)} for come coefficient c.

impact on the datapoints is as follows: https://vp.nnpdf.science/xee4YTRKSAmRmN3iUZfkIg==