PRO dkstats

; Purpose:
;       To accumulate sums of Polar Camera CCD dark frames for the
;	normal range of operating temperatures, with a view to
;	being able to make appropriate dark subtractions from images
;	for which the nearest dark frame is corrupt.
;
; Algorithm:
;       For each dark frame image in the relevant directories, read
;	the header, and extract the date and time.  If these do
;	not appear in the master list of dates/times already
;	processed, read in the file, and the sum images for the
;	integer CCD temperature nearest to that at which the image
;	was acquired, and update the sums.  Then write tham back
;	to disk.
; Necessities:
	- a master list of dates/times already processed (including
	filename and temperature?)
	-