Details:
- Modified the octave scripts in test/3 so that the script does not
choke when one or more of the expected OpenBLAS, Eigen, or vendor data
files is missing. (The BLIS data set, however, must be complete.) When
a file is missing, that data series is simply not included on that
particular graph. Also factored out a lot of the redundant logic from
plot_panel_4x5.m into a separate function in read_data.m.