cardiff: Implementing gnuplot data file generation

When running cardiff on a rampup directory, let's generate a result/
directory that provides the data file for a gnuplot printing.

Inside the result/ directory, you'll find on a job basis the following
files:
- sum.plot
- variance.plot
- variance_percent.plot
- mean.plot

The naming is pretty obvious, it's now time to plot them !