BUG: writing a column with Path objects gives an error with use_arrow #631

Without use_arrow the Path objects are simply serialised to string and written, but with use_arrow this leads to an error like this:

tests/test_predict.py::test_predict_use_cache_skip[False-True-12585] - RuntimeError: ERROR in load_images for footballfields_BEFL-2019_test: ("Could not convert PosixPath('/tmp/pytest-of-runner/pytest-0/test_predict_use_cache_skip_Fa1/sample_projects/_image_cache/BEFL-2019/1024x1024_128pxOverlap/174816/174816_176352_175136_176672_1280_1280.jpg') with type PosixPath: did not recognize Python value type when inferring an Arrow data type", 'Conversion failed for column path with type object')

Reference: orthoseg/orthoseg#287