Add new version of script to combine csv files from VectorPostprocessor

This now uses DictReader and DictWriter.  To be honest, I liked the old version of the script better.  The code was cleaner and it was about 3x faster.  The old one took 3 seconds on my set of 1500 time step data files, while this one takes 9 seconds.  I'm not really sure what the benefit of using DictReader here is.  Maybe the regular python csv reader and writer would be faster.  I've spent too much time on this already, though, so go ahead and merge it if it looks good.