Implement LD-score correction

[Joannavonberg]

Because the effect size of a SNP is partly influenced by the number of SNPs that are in LD with that SNP (i.e. its LD-buddies), we want to correct for this. The LD-score (see LD score regression: https://github.com/bulik/ldsc ) is a proxy for the number of LD-buddies of a SNP. I want to implement a correction for LD-score, on the z-scores of the original GWASs