files are repeatedly downloaded, even if the `file_skip_if_exists`=True

[g4brielvs]

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.

Originally posted by @ArieClaassens in https://github.com/worldbank/blackmarblepy/issues/38#issuecomment-1950989787