add back plot_prefix command line argument
add back plot_prefix command line argument

make default plot_prefix "_"

mit-probabilistic-computing-project/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat,probcomp/crosscat,fivejjs/crosscat,probcomp/crosscat,fivejjs/crosscat,fivejjs/crosscat,fivejjs/crosscat,mit-probabilistic-computing-project/crosscat,mit-probabilistic-computing-project/crosscat,probcomp/crosscat,probcomp/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat,mit-probabilistic-computing-project/crosscat,fivejjs/crosscat,fivejjs/crosscat,mit-probabilistic-computing-project/crosscat,fivejjs/crosscat,probcomp/crosscat,probcomp/crosscat