generic convenience function to convert logbf matrix to PP matrix
logbf_to_pp(bf, pi, last_is_null)an L by p or p+1 matrix of log Bayes factors
either a scalar representing the prior probability for any snp to be causal, or a full vector of per snp / null prior probabilities
TRUE if last value of the bf vector or last column of a bf matrix relates to the null hypothesis of no association. This is standard for SuSiE results, but may not be for BF constructed in other ways.
matrix of posterior probabilities, same dimensions as bf