Add PU value customization to EfficiencyPlotter and EfficiencyCentral

[SanjanShiv]

• Added the ability to customize the PU (Pile-Up) value in the EfficiencyPlotter and EfficiencyCentral classes.
• Default PU value is set to 200 PU, but users can now specify a custom value using the --pu_value argument when running the script.
• Updated the main function to parse the --pu_value argument and pass it to the EfficiencyCentral class.
• Ensured the PU value is displayed on the plots and included in the watermark for better clarity.
• Fixed module import issues by adding the workspace root to the Python path.
  This update enhances the flexibility of the plotting tool and ensures compatibility with different PU scenarios.
  Fixes: Add pileup value to plots #104