examples: make sample scripts cwd-independent

[MohamedHisham20]

Summary

• Makes example scripts runnable from any working directory.
• Resolves config and local data paths relative to each script location.
• Removes cwd-dependent ./cfg and ../phantom assumptions.

Scope

• Updated example scripts in [Sim_Sample.py]
  • [Sim_Recon_Sample.py]
  • [Sim_Recon_Sample_PCCT.py]
  • [Sim_Sample_PCCT.py]
  • [Sim_Sample_Helical.py]
  • [Sim_Sample_XCAT.py]
  • [Sim_Sample_Hybrid.py]
  • [Sim_Sample_Polygonal.py]
  • [Sim_Sample_Analyic.py]
• Added note in [README.md]

Verification

• No remaining ./cfg references in example scripts.
• No remaining ../phantom literal references in example scripts.

[MohamedHisham20]

This PR makes example scripts cwd-independent by resolving cfg/data paths relative to each script.