Update generate_synthetic_density.py naming and validation to be consistent with generate_synthetic_sf.py

[DorisMai]

In PR #234 , generate_synthetic_density.py was refactored to extract common functions for any synthetic data generation. During the review, some minor changes were requested. They are addressed in the new generate_synthetic_sf.py, and for consistency, the density script should be updated too.

Specifically,

1. Improve variable naming occ_mode/values to occupancy_mode/values)
2. Replace boilerplate occ_values validation in __post_init__ with the refactored validate_occ_values() from synthetic_utils.py
3. When validating structure file extension, warn legacy formats by having VALID_EXTENSIONS and LEGACY_EXTENSIONS