PROCEDURE: thm_file_download PURPOSE: This is a general purpose routine will download all THEMIS data files within a given time range. It looks for all procedures that match: "thm_load_*.pro" and executes them with the /DOWNLOADONLY keyword set. Warning - This is a BETA routine and may be deleted in a future release. 23-may-2008, cg, added optional keyword /either to the call to resolve_routine