Add SNP-peak links

In addition to the TF-peak links that we have it would be useful to also have SNP - peak links in the network.

• possible implementation: generate list of relevant SNP-peak annotations outside of pipeline, then provide this additional information to the function that constructs the GRN

This will allow to also keep links in the network that are not necessarily regulated by a TF across the individuals, but by a genetic variant associated to the peak.