readPGM - read in grey-level image in PGM Format.  

Synopsis:string readPGM -> int int int arraytype
fname readPGM -> width height maxval [grayvals]

Description:this function reads an image file in the PGM format and
returns the width, height, maximum gray value and the
image itself (as a linear array).
On Unix systems, man pgm should give you a description
of the PGM (Portable GrayMap) image format.

Parameters:fname - name of file to be read
[grayvals] - one-dim. array containing the pixel gray values,
starting from the upper left corner of the image,
continuing rowwise (normal englissh reading order).
maxval - the maximum gray value
width - width of image in pixels (no. of columns)
height - height of image (no. of rows)

Examples:(FancyImage.pgm) readPGM -> 16 24 255 [grayvals]

This reads the image FancyImage.pgm, and tells you that it has
16 columns, 24 rows, and a maximum gray value of 255. The pixel
gray values are stored in the array.

Author:Schmuker, Gewaltig