Rename write_polar_csv, write_polar_matrix_csv and csv_file_path now that polars can be Arrow #373

Since #372, write_polar_csv and write_polar_matrix_csv write Arrow for an .arrow path, and the geometry YAML key csv_file_path can point at an .arrow polar. The _csv in all three names is now wrong.

All three are public: the two writers are exported, and csv_file_path is part of the geometry YAML schema that BeyondTheSim and other downstream datasets carry. Renaming them is a breaking change, so it is a decision rather than a cleanup.

What I would do: rename the writers to write_polar / write_polar_matrix and the key to polar_file_path. The old writer names would error with a pointer to the new ones. The YAML loader would read csv_file_path as a deprecated alias for one release, so existing datasets keep loading.