PRO extract_header_keyword, header, keyword, keyval, keytype, status, errstr

; Description: This module extracts the value and type of a header keyword specified by 
;              "keyword" from a string array "header" containing an image header. If the 
;              header keyword is present and unique in the header, and the value of the 
;              header keyword is extracted successfully, then the module returns the 
;              value of the header keyword as the parameter "keyval", the IDL type
;              of the keyword as the parameter "keytype", and sets the value of the 
;              parameter "status" to "1". If the header is missing the required header 
;              keyword, or the header has multiple entries for the required header 
;              keyword, then the module returns a value of "0" for the parameter "status"
;              and an error string via the parameter "errstr".
;
; Input Parameters:
;
;   header - STRING VECTOR - A vector containing the entries of the image header.
;   keyword - STRING - The header keyword whose value is to be extracted.   
;
; Output Parameters:
;
;   keyval - SAME TYPE AS IN HEADER - The value of the required header keyword.
;   keytype - INTEGER - The IDL variable type code of the value of the required header
;                       keyword.
;   status - INTEGER - If the header keyword is present and unique in the header, and 
;                      the value of the header keyword is extracted successfully, then 
;                      the module sets the value of the parameter "status" to "1". If 
;                      the header is missing the required header keyword, or the header 
;                      has multiple entries for the required header keyword, then the 
;                      module returns a value of "0" for the parameter "status".
;   errstr - STRING - If the parameter "status" is returned with a value of "0", then
;                     "errstr" is returned with the corresponding error string.
;
; Author: Dan Bramich (dan.bramich@hotmail.co.uk)
;
; History:
;
;   14/01/2009 - Return the header keyword value IDL variable type (dmb).
;   19/05/2008 - Module created (dmb)

;Check that "header" is a non-empty string vector

;Check that "keyword" is a non-empty scalar string

;Extract the header keyword from the header

;If the keyword is missing from the header, then return an error

;If the keyword is duplicated in the header, then return an error

;Set "keyval" to the value of the header keyword, set "keytype" to the corresponding IDL 
;variable type code, and set "status" to "1"