;+
; NAME:
;	CCG_MEANS
;
; PURPOSE:
;	Calculate annual, monthly, or 
;	daily averages.  Return arrays
;	of averages, number and standard
;	deviation. 
;
; CATEGORY:
;	Miscellaneous.
;
; CALLING SEQUENCE:
;	CCG_MEANS,	xarr=xarr,yarr=yarr,year=1,xres,yres,sd,n
;	CCG_MEANS,	xarr=xarr,yarr=yarr,month=1,xres,yres,sd,n
;	CCG_MEANS,	xarr=xarr,yarr=yarr,day=1,xres,yres,sd,n
;
; INPUTS:
;	xarr:		Decimal date array.
;	yarr:		Corresponding values.
;
; OPTIONAL INPUT PARAMETERS:
;	year:		If non-zero calculate annual averages.
;	month:		If non-zero calculate monthly averages.
;	day:		If non-zero calculate daily averages.
;
; OUTPUTS:
;	xres:		Decimal date array for annual, monthly, or
;			daily averages.  Interval is centered, i.e.,
;			1985.5.	
;
;	yres:		Corresponding averages array.
;
;	sd:		Array of standard deviations of the
;			the calculated averages.
;
;	n:		Number of individual values used in 
;			determining average.
;
; COMMON BLOCKS:
;	None.
;
; SIDE EFFECTS:
;	None.
;
; RESTRICTIONS:
;	None.
;
; PROCEDURE:
;	CCG_MEANS may be called from the IDL command line or
;	from an IDL procedure.
;		Example:
;			CCG_FREAD,	$
;			file='/projects/ch4/in-situ/mlo_data/year/mlo93.ch4',nc=6,var
;			;
;			CCG_DATE2DEC,	$
;			yr=var(1,*),mo=var(2,*),dy=var(3,*),hr=var(4,*),dec=dec
;			;
;			CCG_MEANS,	$
;			xarr=dec,yarr=var(5,*),month=1,xres,yres,sd,n
;			.
;			.
;			.
;
;		
; MODIFICATION HISTORY:
;
;	Written, KAM, March 1994.
;-