Wrote function _check_download_file() to download GOES data files
to user's download directory if not already there.
Added kwargs to all relevant functions() to force the GOES data files
to be downloaded even if they're already there.
Also created variables for the various GOES data files at the start
of goes.py defining names of GOES data files and replaced usage of
these file names with the relevant variable.
Changed order of imports to be in line with PEP8 convention.
Changed way _check_download_file() deals with default of remotename kwarg.