function thm_valid_level purpose: for thm_load routines to validate the level keyword and set it to a standard format ('l1', 'l2', etc.). Only a single level is allowed. keywords: level: level keyword input from thm_load routine vlevels: a space-separated string, like 'l1 l2' deflevel: a string like 'l2'. defaults to 'l1' example: lvl = thm_valid_level('l1','l1 l2','l1')