allowing for 2d contour plots comparing two different set of chains

changes in detail:
- added `-plot-2d` command-line argument to enforce plotting of the triangle plot
- can be used to choose plotting (filled) contours on top of each other,
  or the sencond set of contours is overplotted as contour line
- several changes to analyze.py to enable triangle plots in -comp mode
- output of comparing 2d plots: <chain1>-vs-<chain2>_triangle.pdf