fix: interpolate invalid GPS timestamps

[ptpt]

This pull request introduces several improvements to the mapillary_tools/geotag/gpmf_parser.py file, focusing on enhancing GPS timestamp handling and code organization. The most important changes include reordering imports, adding error handling for GPS timestamp conversion, implementing a function to backfill GPS timestamps, and modifying the extraction of points from samples to use the new backfill function.

Enhancements to GPS timestamp handling:

• Added error handling in the gps5_from_stream function to ensure robustness when converting GPS timestamps.
• Implemented a new function _backfill_gps_timestamps to fill in missing GPS timestamps based on available data points.
• Modified the _extract_points_from_samples function to use the _backfill_gps_timestamps function, ensuring that GPS timestamps are backfilled both forward and backward from the first available timestamp.

Code organization improvements:

• Reordered imports to follow standard conventions, moving the datetime import to the correct position.