Comp 2d contour plots

see previous pull request from comp_2d_contour_plots branch:

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

changes compared to previous pull request:
* 2d contour plots in comparison mode now without tabulators and the correct number of intendation spaces