Improved argument parsing and error handling in __main__.py

[SophiaLi20]

This pull request improves the command-line interface for the main.py script in the AtomDB project. The following changes have been made:

Improved Argument Parsing:

Replaced args.compile and args.query with the correct argument names args.compile_species and args.query.
Ensured better clarity in help messages for arguments and commands.
Error Handling:

Added input validation for positive integers for charge and mult arguments.
Ensured exc is non-negative, providing meaningful error messages when invalid values are passed.
Code Modularity and Clarity:

Encapsulated the main logic within a main() function for better structure.
Added comments and docstrings for better readability and maintainability.
User Feedback:

Added informative messages to guide the user during compile_species and query operations.
Benefits:

• Enhances user experience with clearer error messages and help descriptions.
• Improves code maintainability and readability for future contributors.
• Ensures robust input validation to prevent runtime errors.

These changes make the script more reliable and user-friendly for both developers and users