AVG

       Return the average value of an array, or 1 dimension of an array

       Calculate the average value of an array (in which case AVG is identical
       to the RSI procedure mean.pro), or calculate the average
       value over one dimension of an array as a function of all the other
       dimensions.

       RESULT = AVG( ARRAY, [ DIMENSION, /NAN, /DOUBLE ] )

       ARRAY = Input array.  May be any type except string.

       DIMENSION = Optional dimension to do average over, integer scalar

      /NAN - Set this keyword to cause the routine to check for occurrences of
            the IEEE floating-point value NaN in the input data.  Elements with
            the value NaN are treated as missing data.
      /DOUBLE - By default, if the input Array is double-precision, complex,
                or double complex, the result is of the same type;  64 bit
                integers are also returned as double.   Otherwise the result
                the  result is floating-point.   Use of the /DOUBLE keyword
                forces a double precision output.   Note that internal
                computations are always done in double precision.

       The average value of the array when called with one parameter.
       If DIMENSION is passed, then the result is an array with all the
       dimensions of the input array except for the dimension specified,
       each element of which is the average of the corresponding vector
       in the input array.
       For example, if A is an array with dimensions of (3,4,5), then the
       command B = AVG(A,1) is equivalent to
                       B = FLTARR(3,5)
                       FOR J = 0,4 DO BEGIN
                               FOR I = 0,2 DO BEGIN
                                       B[I,J] = TOTAL( A[I,*,J] ) / 4.
                               ENDFOR
                       ENDFOR

       Dimension specified must be valid for the array passed; otherwise the
       input array is returned as the output array.

       AVG(ARRAY) = TOTAL(ARRAY, /DOUBLE)/N_ELEMENTS(ARRAY) when called with
       one parameter.

       William Thompson        Applied Research Corporation
       July, 1986              8201 Corporate Drive
                               Landover, MD  20785
       Converted to Version 2      July, 1990
       Replace SUM call with TOTAL    W. Landsman    May, 1992
       Converted to IDL V5.0   W. Landsman   September 1997
       Added /NAN keyword   W. Landsman      July 2000
       Accept a scalar input value    W. Landsman/jimm@berkeley   November 2000
       Internal calculations always in double precision W. Landsman March 2002
       Return NAN if all values in array are NAN  W. Landsman April 2002
       Fixed coding bug if all values in array are NAN W. Landsman Jan 2004

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