Plotting changes

- [x] **Important**: fixed a bug with the error bars showing fake errors (critical for last bins of main x-section plots)
- [x] Fixed the memory leak in the plotting script. Now doesn't consume more than 200Mb RAM
- [x] Added an exception for empty v+jets template (happpens in >=2m bin) in get_fit_results
- [x] Latex labels for systematic plots, smaller legend so data points are visible
- [x] Proper sorting in legends
- [x] Now can produce template plots for all variables even if the templates are zero
- [ ] Still need to make last bins with small errors visible (zoom? log plots?)