Add comprehensive testing infrastructure and remove SPHEREx_Simulator_Tools dependency

- Add proper package imports in __init__.py for easy access to Tabular_Bandpass
- Remove hard dependency on SPHEREx_Simulator_Tools, making package self-contained
- Implement fallback data_filename function for local data file access
- Add graceful error handling for missing DBS data files
- Create comprehensive pytest test suite:
  - test_import.py: Basic import functionality tests
  - test_basic_functionality.py: Class structure and optional dependency tests
- Add pytest configuration in pyproject.toml and pytest.ini
- Add development dependencies (pytest, pytest-cov, black, flake8)
- Create example script (ex.py) demonstrating basic usage
- All import tests now pass successfully