Fix skip if exists

[g4brielvs]

The file_skip_if_exists was removed and needs to be reimplemented.

[ArieClaassens]

Thanks for the great work, but files are repeatedly downloaded, even if the ile_skip_if_exists=True parameter is passed in the sample code with the bm_raster function, and file_directory destination is provided. Inspection of the download.py module doesn't show any logic to check for existing local files.