BVG-PLT:  Get quantities from a bvg log file, in a form suitable to plot.

The plot program for bivariate Gaussian distributions is just the
generic plot module (for which, see xxx-plt.doc), with no new
quantities defined, except that an application argument of "ellipse"
may be specified, in which case a one-standard-deviation ellipse is
plotted for each log file.  Doing this makes sense only if the rest of
the plot is of the values for the two variables (q0 and q1).

            Copyright (c) 1995 by Radford M. Neal