sim_betas: Generate effect sizes betas
In KaiqianZhang/prsim: Simulations for PRS Analysis

Generate effect sizes betas

1	sim_betas(seed = 1234, M, p, h2, maf_vec, prare, dist, ld)

`seed`	random number seed
`M`	the number of snps
`p`	the proportion of causal snps out of all snps
`h2`	heritability
`maf_vec`	the vector of MAF for all snps
`prare`	proportion of rare causal snps out of causal snps
`dist`	distribution of effect sizes, default is "gaussian". Options can be "gaussian", "exponential", "laplace". Note that LDpred simulates non-zero effect sizes from laplace distribution.
`ld`	linkage disequilibrium. Options can be "strong" or "weak". We simulate nearby snps together when ld is strong. We simulate snps evenly spread over all variants when ld is weak.