Additional extension fields and option to extract CSV

[yushiyangk]

Hi, thanks for this very useful project. I have implemented some additional features for my own use, that I hope you would consider incorporating upstream.

Summary of changes:

1. Added names for some of the protobuf fields that were previously labelled as 'unknown', and included them in the extracted gps_data
2. Expose extract_extensions to the CLI with a new option --additional
3. Added an option require_gps=True, which when set to False allows points to be extracted even when GPS data is missing
4. Added a method OsmoGps.save_csv() and CLI command extract-csv in addition to gpx

Do let me know if you have any thoughts, thank you!

Backwards compatibility

There should be no breaking changes.

The added parameters (require_gps) and added CLI options default to their previous behaviour.

Details

For (3), save_gpx() and resample() both will raise warnings if require_gps was set to False (defaults to True). When True, any data points that do not have GPS information will raise a warning, similar to prior behaviour.

Also, resample() will automatically discard any data points without GPS information if resampling_method is not none, because it's not clear to me how such data points should be treated otherwise.

For (4), I moved some of the duplicated logic between save_gpx and save_csv into their own functions. Output formatting is now done in _render_point(). I also made main.py:extract() take save_method as an argument.