FUNCTION str2chars, str, STR_INDLO = str_indlo, STR_INDHI = str_indhi

; Description: This function splits a string "str" into a vector of characters, with
;              the option of only extracting the characters corresponding to a 
;              substring between the string position indices "str_indlo" and 
;              "str_indhi".
;
; Input Parameters:
;
;   str - STRING - The string for which the characters are to be extracted.
;
; Return Value:
;
;   The function returns a vector of STRINGS where each element is a string consisting
;   of one character. The ith element of the return vector corresponds to the ith
;   character in "str". 
;
; Keywords:                              
;   
;   If the keyword STR_INDLO is set to an INTEGER (or LONG) value, then only the
;   characters at or after the string index position specified by this keyword will
;   be extracted. If this keyword has a value that is out of range for the input
;   string "str", then it will be ignored.
;
;   If the keyword STR_INDHI is set to an INTEGER (or LONG) value, then only the  
;   characters at or before the string index position specified by this keyword will
;   be extracted. If this keyword has a value that is out of range for the input
;   string "str", then it will be ignored.
;
; Author: Dan Bramich (dmb@ing.iac.es)
;
; History:
;
;   14/01/2009 - Module created (dmb)

;If "str" is not a scalar string, then return an empty string

;If "str" is an empty string, then return an empty string

;If the keyword STR_INDLO is set, then check that it makes sense

;If the keyword STR_INDHI is set, then check that it makes sense

;Extract the required vector of characters from the input string

;Return the required vector of characters