FUNCTION dir2root, filename

; Description: This function takes a file name with a full directory path and
;              returns the directory path without the file name.
;
; Input Parameters:
;
;   filename - STRING - The full directory path and file name to be processed.
;
; Return Value:
;
;   The input parameter "filename" is returned with the file name removed. For
;   example, for the filename '/data/dmb/ngc7789.fits', the function would 
;   return the string '/data/dmb/'. 
;     In more detail, if the parameter "filename" is not a string, then the   
;   function returns an empty string ''. If the parameter "filename" does not 
;   contain any instances of the character '/', then the function also returns
;   an empty string ''. If the parameter "filename" does not start with the
;   character '/', then the function also returns an empty string ''. If the
;   parameter "filename" starts with the character '/', then the function
;   returns the substring in "filename" starting with the first character of
;   "filename" and ending with the last instance of the character '/'.
;
; Author: Dan Bramich (dmb@ing.iac.es)
;
; History:
;
;   19/05/2008 - Module created (dmb)

;Check that "filename" is a scalar string

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

;Decompose "filename" into individual characters

;Find the occurrences of "/" in "filename"

;If "filename" does not contain any instances of the character "/", then
;return an empty string

;If "filename" does not start with the character '/', then return an empty 
;string

;If "filename" does start with the character '/', then return the substring 
;in "filename" starting with the first character of "filename" and ending 
;with the last instance of the character '/'.