Retrieve LD proxies
Arguments
- dat
 Simulation object produced by `sim_mv`
- index
 list of indexes to retrieve proxies for
- R_LD
 LD pattern used to generate dat
- r2_thresh
 Get proxies with r^2 >= r2_thresh with one of the index variants
- return_mat
 If TRUE, return the correlation matrix between the index variant and the proxies. In this matrix, the the index variant always corresponds the first row/column and the proxies are in the order returned.