function thm_valid_input purpose: for thm_load routines to validate the input keyword and set it to a standard format ('l1', 'l2', etc.). parameters: input: keyword input to be validated from thm_load routine: array or scalar label: name of input keyword that is being validated keywords: vinputs: a space-separated string, like 'l1 l2' definput: a string like 'l2'. Required. include_all: if set will accept multiple values on input, and include 'all' as a valid input. 'all' is equivalent to '*' verbose: to maintain control of verbosity format: format to use to convert numerical input to string. Numerical input disallowed if absent. examples: "('l',I1)" or "('v', I02)" return value: a scalar string, e.g. 'l1' if /include_all: an array of strings on error: empty string: '' example: lvl = thm_valid_input(level,'Level',vinputs='l1 l2',definput='l1',$ foramt="('l', I1)", verbose=verbose) if lvl eq '' then return