preproc overwrite doesn't fully overwrite

in any of the preproc wrappers, the `overwrite` flag allows you to write to a file that already exists, but it doesn't mean that the file will be overwritten, just that the contents will be appended. currently it's just a safety mechanism for not accidentally changing files you don't want to. but if there is outdated information in the file, e.g. eye calibration, then re-doing the preprocessing with `overwrite=True` doesn't remove that calibration.

related to #252