as well as uses pathlib and some comment corrections

…ncation

2) Changed the number of bins to 200 for better accuracy
3) added some printouts for clarity

- Introduced `read_photon_data` function to encapsulate the logic for reading and processing photon data from OpenPMD series iteration.
- Introduced `read_electron_data` function to encapsulate the logic for reading and processing electron data from OpenPMD series iteration.
- Refactored the `main` function to use the new abstraction functions for reading photon and electron data.

This change improves code readability and maintainability by separating concerns and making the main function less complex.

 - in np.interp() added left=0 and right=0
 - subtract_functions() is now relative_error_array()
 - SystemExit changed to ValueError

and reverted the ValueError to SystemExit