New features for matrices and plotting

This release includes fixes to concatNext and the new table2nexus which reads data in plain-text value-delimited format (e.g., tsv, csv, tab) and returns the data in nexus format. It also includes the function crossplot which allows to compare pairs of densities (e.g. prior vs. posterior or independent runs of the same analysis) and plot the median and error bars representing the highest density interval, these around the identity line y = x.