Add catch for empty log_evidence_err array
I ran into an issue when trying to use the ResultList object where all the log evidence errors in my individual runs were infinite so I wasn't able to use the combine() method. I've just added a catch that if this array is empty, return nan as the new result.log_evidence_err. I could also see the value in returning the standard deviation of the evidences from the individual runs. What do the rest of you think?