function FILE_CHECKSUM Purpose: Returns: SHA1 CHECKSUM string for a file Usage: output = file_checksum( filename, [/add_mtime] ) Typical usage: file_hash( file_search('*') ,/add_mtime ) ; Origin: Mostly copied from file_touch Limitations: Currently works only on Linux and MacOs systems by calling the shasum command in a shell. This module is under development. Author: Davin Larson (davin@ssl.berkeley.edu) copyright - March 2014 Changes: December 2014 - Name changed from file_hash to file_checksum because is better reflects its true purpose License: All users are granted permission to use this unaltered code. It may NOT be modified without the consent of the author. However the author welcomes input for bug fixes or upgrades.