Internal function, calculate posterior probabilities for configurations, given logABFs for each SNP and prior probs
combine.abf(l1, l2, p1, p2, p12)
merged.df$lABF.df1
merged.df$lABF.df2
prior probability a SNP is associated with trait 1, default 1e-4
prior probability a SNP is associated with trait 2, default 1e-4
prior probability a SNP is associated with both traits, default 1e-5
named numeric vector of posterior probabilities