fix shape bug and add PostFitShapes program

PostFitShapes:
  * takes a datacard as input and writes pre- and post-fit histograms of each process to an output file, as well as total signal and total background histograms.
  * It is then up to the user to write some plotting script using these shapes
  * the program can also print out the shift in normalisation of each process after the fit, and the relative uncertainty on the total background. This can then be used by analysts to make post-fit control plots
  * `./bin/PostFitShapes --help` gives the usage